As our Cougars and Thunderbirds age, you're going to run into repairs that are quite abnormal. The rear axle is one of those areas that mystify a lot of owners, and confound those that work on them. There are quite a few key components in the system that we're going to cover below, so that you can make an informed decision and know that semantics—and the cost—beforehand.
All Dana-sourced rear axles used on these cars left the factory with a metal tag installed on a differential cover bolt, and that tag included codes for the rear end size, gear ratio, differential type (locking or open), and production date. You can see the chart at this site to decode the information. If your tag is missing or broken off, or is rusted so badly that you can't read it anymore, you can still look on the driver's door tag for the axle code character and decode it using this link.
We've discussed the rear axle housing situation a few times before (see here for more info), but in a nutshell, there were three different width axle housings used on our cars between 1983-88, and fortunately they're well defined:
NOTE: These figures refer to the bare axle housing only, NOT the overall width of the rear axle with axle shafts and brakes installed! Please verify your housing width vs. the new one, as there can be slight variations in widths.
If your housing somehow got bent or damaged and you need another one, the information above is what you'll need to get a replacement. We haven't really run across "new" housings but there seem to be good, used bare ones available. It's also possible to use an aftermarket housing but it likely won't appear as stock.
What defined the overall width of the rear track was the length of the axle shafts. Again, that's been covered in this section here. Our cars had a unique axle shaft width that really wasn't shared with any other 4-lug Ford passenger car of the era. That's not necessarily a bad thing, but something of which you'd need to be aware if getting a new shaft. This width axle shaft was consistent, regardless of the housing or model year. All factory axle shafts were 28-spline.
So why would you need a new axle shaft? Well, over time they can rarely warp (usually from excessive heat or low fluid), or maybe you accidentally hit a curb too hard with the rear tire, or maybe you've been in an accident. Any of those can twist up the axle shaft to the point where it's too wobbly to reuse. Or, it could be that the rear bearings have worn the shaft out so badly that even "axle saver" bearings (which move the bearing contact points) cannot be used. A bent axle is an unsafe condition and should never be used on a daily basis!
Rear axles can be purchased through parts stores, Summit Racing, eBay, Amazon, etc. but be sure you have the correct length! Check that the new axles have a good warranty. If you can only find a performance-brand axle, that's okay as well, so long as it's 28-spline and has the correct 4-on-4.25" bolt circle. It would be very wise to get new bearings as well.
NOTE: You will need to find your car's manufacturing date on the driver's door tag and use it to align with the following information:
1983 to Feb. 1, 1985: The shaft needs to measure 30-1/2" from the end of the spline to the outside stud face. The original Ford part number is E3SZ-4234-A. The same axle shaft was used for both sides of the 7.5" rear end (there were no R/L specific lengths).
Feb. 1, 1985 to 1988: The shaft needs to measure 29-9/16" from the end of the spline to the outside stud face. The original Ford part number is E5SZ-4234-A. The same axle shaft was used on both 7.5" and 8.8" rear ends, and for both sides of the rear end (there were no R/L specific lengths).
Above: 1983-mid 1985 9" brake drum with raised hat section.
Above: Mid 1985-1988 9" brake drum with flat hat section.
If your have a 1985 car and the build date is close to February or March 1985, you can have either width rear axle. That's due to the supply of the older vs. newer width rear ends as they got used up on the assembly line. To quickly be able to tell which one you have, safely jack up the car and remove a rear wheel. Earlier rear ends used rear drums with a raised "hat" section in the center; later cars had no raised section. This applies to both the standard 9" drum as well as the optional 10" drum.
If you hear a howling sound from the rear axle as you drive, it could be that the ring and/or pinion gears are going bad. They generally last a long, long time but can fail for lots of reasons (low fluid, excessive heat, accident, etc.). New Ford ring and pinion sets are available and we'd recommend the Ford gears over most aftermarket ones, simply because they're engineered for these cars and they tend to fit a lot better. Most of these cars had the 7.5" rear end with either a 2.73 or 3.08 gear ratio and an open differential. Turbo-4 cars could be optioned with 3.45 gears and Traction-Lok (limited slip). The 1988 5.0L cars had the 8.8" rear end and 3.08 gears. We have never found any lower (numerically higher) factory gears for that rear end. Please refer to the rear axle chart here to decode your gear ratio.
While the gears are getting replaced, it would be very wise to install new shims. These shims make sure everything is where it should be, with no excessive play in the gears. Sometimes the old shims can be reused, and if they're in good shape, that's okay but we suggest planning on a new shim kit just in case they're needed.
Cars that have an "L" in the lower lefthand line of the rear axle tag are equipped with a limited-slip differential, which Ford called Traction-Lok. (This can also be verified on the door tag using the decoder here). In normal situations this allowed both axles to engage simultaneously for better off-the-line traction and power application to the ground. It was an option on most of the lower-model cars and standard on the XR7 models, for both the 7.5" and 8.8" rear axles. Cars with the optional towing package usually had this option as well. The locking 7.5" unit is considered to be somewhat more uncommon in these cars.
If you suspect that the Traction-Lok differential is not behaving properly, such as excessive slipping or noise, please have the unit inspected. With age and usage, it's common for these units to begin failing and be in need of replacement. Stock-type units are available pre-rebuilt for easier installation. Also be sure to have the clips (S-clip in the center, C-clips for holding the axles in) inspected for uniformity and to make sure they're not bent or missing.
The stock rear differential cover is stamped steel and is identical to nearly every other Ford passenger car of the era. Nothing special here. None had an external drain plug. These can be found new, used, in salvage yards, eBay, etc. so they're plentiful enough to find should you need one.
Ford specified a 75W-90 weight gear oil for these cars. You can run any quality rear axle fluid so long as they meed all standards and specifications. For those cars with a locking (limited slip) differential you MUST run Ford friction modifier in the rear fluid! It's available through the Ford dealer or online. Fluid doesn't necessarily need changed often, but if you do a lot of towing, long trips, or climb a lot of hills, it may be a good idea to change the fluid more often than the recommended 75,000 miles. All 7.5" rear ends used 3.5 pints of fluid, and 8.8" rear ends used 3.8 pints.
It's usually not necessary to use synthetic fluid unless your car meets severe use conditions (constant low or high speeds, towing, excessive idling, etc.) but should you wish to use synthetic, it's safe to do so.