While changing the oil today I crawled under the rear axle and checked the rear diff fluid level. Levels were good, but the fluid was a kind of grayish sludge (might be a pointer to my decreasing fuel economy). I headed down to the parts store because I figured I'd need a new gasket and obviously new gear oil. They said there wasn't a gasket for my truck and they weren't sure about the oil either. They said that the type of gear oil and amount depends on the type of axle I have. I told them I'd call the dealer and see if they can give me the info on my equipement based on the VIN.
Is this true? I figured I'd just crack the cover open and drain it, then tighten it back up and fill it with gear oil. Maybe I'm too simple...

 

The gaskets on the front and rear differentials are reusable. Just be sure to wipe the mating surfaces off with a clean rag. As far as differentials go, AAM calls for 75w/90 GL5 differential fluid. The rear 11.5 diff. will take 122 oz. or 3.8 quarts and the front 9.25 will take 76 oz. or 2.375 quarts. However, some early third gen models came with a 10.5 AAM rear diff.

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My truck is a 2005 2500. Is there any way to tell what I've got for axles?

 

If your truck is 4wd it's 9.5" front and 11.5" rear. Both from AAM.

MikeyB

 

Make sure the gear oil is GL-5 Synthetic. No additive is needed for the limited slip.

 

From the parts store, Mobile One 75w90 Synthetic is your best option. I run Amsoil, but I run Mobile One in my other vehicles.

 

Thanks everyone for the help. Got it all swapped out without incident.