Need a crash course in differentials

My knowledge is way to short on this subject and I have tried to piece things together from the various posts.
Basics. Went to replace bearings & oil seals on rear axles on my 74'. After getting "wrong" parts a couple times it seemed that the rear diff. may not be the stock one anymore, unfortunately no tag to ID. Went to replace rear wheel cylinders and again same issues, parts that should be the stock ones weren't matching up. After alot of part searching at the local auto house it seems that the PO put in a rear diff from a Ford truck.

This is what I know:
Front diff has a tag 3.54 D4TA-ACA 603471-3 = 74' Dana44 3.54 gear ratio?
Rear no tag, case mark C5AW 4668A = NEEDS TO BE SAME GEAR RATIO? The code puts it in 9" rear end class as far as I can tell.
When I pulled out the axles they were 28 spline and one axle clearly had come from a salvage yard b/c it was marked in yellow grease pen "75 Bronco, 28 spline". I am guessing I will need to open up the differential to know any more specific info regarding what this differential is from whether it is stock or from another rig?

I bought a parts 73' with a full drive train but will keep that to the side for now until I can figure out what is currently equipped in my rig.

Any suggestions would be greatly appreciated.

If it was a truck rear than it would have a wider track than the bronco, by about 6 inches. In 75 the rears had a big bearing but small brakes option and that required a specific axel. If you have the big bearings but 10 inch brakes than you have identified your axel.

Your axles need to be geared the same if you plan to use 4wd at all. The 9" is easy to work on (for figuring out what you need to). With the axles out, you can remove the center section and it comes apart as one piece. From there you can count the ring teeth and pinion teeth and divide those out to calculate the ratio.

If all you are trying to do is replace the axle bearings and seals, there is no need to remove the differential. That won't tell you anything about the axle parts. Try reading through the rear axle section on Wildhorses website. It should give you plenty of info to determine what you have.

http://www.wildhorses4x4.com/category/bronco_rear_axle

The inside diameter of the Bronco rear brake drum is approximately 10" or 11". So, if you have a large bearing and a 10" brake drum, you have what we call a 74-75 medium duty setup. That leaves us with the 66-75 large and 76-77 large. Both use an 11" brake drum. The 66-75 uses a narrower drum and corresponding brake shoes. We identify this as 11 X 1 3/4". The 76-77 uses an 11 X 2 1/4" drum and shoe.

Order your bearing from a vendor, the auto parts store is too dumb to figure it out.

hc37180 said:

After getting "wrong" parts a couple times it seemed that the rear diff. may not be the stock one anymore, .

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I agree with the other posters..your chances of getting the wrong parts for your unmolested EB are about 100%...from the usual criminals at the parts houses.

Consider this..not once ever..has one of those parts chains ever managed to offer me the correct u-joint..axle seal..bearings..

Welcome to the world of owning a catalog rig in the computerized part selection age!;D

"Back in the day" when the mom-pop stores were still around and I dealt exclusively with those, I would walk in the door and ask for a part and the old man running the place would look me in the eye and say: "iffen you didn't bring me a part number, you just git your butt behind this catalog rack and look it up yrself". He knew that a) Ford part lookup is a nightmare ..always has been, and b) if I got it wrong..it wouldn't be his fault.;D

Pa Pitt knows exactly what I'm talking about.

thegreatjustino said:

If all you are trying to do is replace the axle bearings and seals, there is no need to remove the differential. That won't tell you anything about the axle parts. Try reading through the rear axle section on Wildhorses website. It should give you plenty of info to determine what you have.

http://www.wildhorses4x4.com/category/bronco_rear_axle

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Thanks for the info, I was only thinking of that as an option for trying to ID the gear ratio for the rear end because of the missing ID tag. I will check out that section at wild horses. I had not come across it yet. Great info as always and extremely appreciated.

rjrobin2002 said:

The inside diameter of the Bronco rear brake drum is approximately 10" or 11". So, if you have a large bearing and a 10" brake drum, you have what we call a 74-75 medium duty setup.

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This is exactly the type of information I was looking for, basically does it seem plausible that my setup could actually be stock or found in my year Bronco. When I couldn't find the tag on the rear diff I was thinking here we go again.
Thank you very much.