Differential Maintenance

Just looking at my differentials and comparing them to pics I see around here I've a D30 in front and a C8.25 in the rear. I'm sticking with these for now but I'm wanting to do some work and I've some questions.
I want to change the fluids just to ensure its fresh as I don't know when it was done last. My D30 is leaking from around the cover seal so when I change the fluid I'll fix that.
What fluid would you recommend for these? I've seen a multitude of responses to this question, but curious on brands and types. Can I use the same in both?
How many quarts should I buy to complete the job?
When I open these up how can I tell what my gearing is? What about splines? Probably simple but I've never worked on a diff before so I'd like to get a better idea of what I have.

Lastly, any tips? I plan on just taking the covers off, cleaning out the old fluid, putting the cover on with RTV and topping off the fluid. Seems easy enough, am I an idiot and missing something from what I've read?

Order lube locker gaskets. You can thank me next time you have to change the fluid.
5qts of 75-140 should do it
for ratio, it should be stamped in there. If not divide ring gear teeth by pinion teeth.
as for spl count, you would have to pop a c.clip and pull the axle to know for sure.

Assuming it's stock, and a 4.0, you should have 3.07 gears if it's a manual, and 3.55 for an automatic. A 99 should have the 29 spline rear axle, IINM.

agreed on all above... as long as it's factory and hasn't been put together from junkyard parts that is all correct.

Lube lockers are awesome.

The easiest way to get the old RTV off the cover is with a brass wire wheel, it comes off the diff housing pretty easily with a razor scraper but the covers are contoured on the edges sometimes and trying to do it with a scraper is annoying.

Find out if the rear has a factory limited slip diff, because if it does you'll have to have oil that has a friction modifier or add a friction modifier fluid with the diff oil. If not, then normal oil is fine

Thanks tons, guys. Theres a sticker on the axle that says 3.55 and since I got it in what appeared stock I believe the 3.55.

$20 or so for lube lockers for the ease and good seal, it'll be worth it. Still have to lift her up and spin the wheels to see if it has an LSD. I've read that sometimes there's a sticker in the glove box that let's you know but it wasn't there.

You dudes are awesome!

souske said:

Order lube locker gaskets. You can thank me next time you have to change the fluid.
5qts of 75-140 should do it
for ratio, it should be stamped in there. If not divide ring gear teeth by pinion teeth.
as for spl count, you would have to pop a c.clip and pull the axle to know for sure.

Click to expand...

Unless he has a Class III hitch and the towing package--and is towing with his XJ--75w90 would be appropriate gear oil front and rear.

I rhought the thicker oil was preffered for towing/ heavy duty use

I believe Joe is saying that the 75W90 is fine, unless he is towing with it.

It reads that way now that you say that.
the booklet that came with my arb seems to corroborate my heavier weight info.
75-90 for dd use
75-140 if you abuse it

I can use the 75-140 in the summer when I haul the pop up and 75-190 in the colder months when I'm not hauling. Seems often but 2x a year will be easier with the lube lockers and if it ensures the life of my diffs.

I use amsoil 75w-110 front and rear

I run the cheapest Coastal 75w140 that autozone has... no issues yet.

Figured I'd either be regearing or swapping to bigger axles (or both) within 50k miles, so anything that meets OEM spec is probably fine.

Once a year is more than enough, unless you drive a crazy amount of miles

Heck I think 50k is the change interval for most mfgs