My car is STILL making a loud noise from the rear, and the only thing left is the differential. How do I locate where to check the oil level? check the level of the oil? Drain the oil? What oil to I buy to replace the oil? Am I better off going to Jiffy Lube or some place like that?

What model year car??? Changine fluid usually involves a teardown. If the diff is not showing any leaks it's probably not leaking. If it is leaking there is a 1/2" square drive plug normally in the back of the diff. It can be a pain to get to on some cars. But it might need to be rebuilt.

Sorry, it's an 1989 xj6. Please describe EXACTLY where the differential is located. I've already looked for it 3 times, and it's a pain lifting the car.

Directly between the rear wheels.
Follow the stub axles from the back of the wheels (where you puit the new axle bearing).
The differential is between the stub axles.

Main driveshaft (from the transmission) fits into the front of the differential.

Been many moons since I did one, but I think you need to pull the back cover to drain (big mess), clean, re-seal, and re-fill.

If there is a drain plug (hopefully) at the bottom, pull it and drain it.

I have yet to see a Jaguar diff with a drain plug. and most of the diffs do not have the rear pans to undo.

According to the SNG Barrett Parts Catalog, XJ6/12's have a 607172J drain plug, AND a 3931 rear cover gasket, cover held on by 10 or 12 bolts, can't tell.

Mark Two's, drain plug P/N3823, gasket for cover P/N3931 using 10 bolts to affix.

Yet to see a jag with a drain plug?
MOST diffs do not have covers?

E-type, once again, SNG Barrett catalog, drain plug P/N 607172J (same as the XJ6), COVER gasket P/N 3931 (again, same as XJ6).

Thought I would do a bit more research, since I hadn't done this work in 20-30 years.

XK-120's, early, ENV axle, pull the pumpkin out the nose after pulling the axle shafts.
Later 120's, 140's, and 150's, Salisbury axle, use various gaskets on the rear cover, P/N 802, P/N 3847, and our perennial favourite, P/N 3831.

SNG Barrett lists a (!) 607172J drain plug, not specified if some had it and some did not.

So much for doing it myself, I'm taking the car to
Eco Lube tomorrow. They said they'd do it for $25, and
since the auto parts stores don't seem to carry the oil I
need, and Eco does, they are a perfect candidate. Thanks for your help. Hopefully next time I can do it on my own.

Is this a limited slip diff?
They have the additive?

The XJ40 has both a drtain plug and a fill plug. The fill plug is inconviently located on the back cover of the diff. It is either an "innie" or an "Outie"square drive plug. It is either 9/16 square, or 1/2 square.
The drain plug is , oddly enough, on the bottom. I think it is accessed through a hole in the lower "pan" on the diff carrier. It is usually the same as the fill plug.

ALWAYS remove the fill plug first. In case you do not have the correct combination of tools to get the often tight plug loose, you don't want to have already emptied the diff....

Any 85w 140 gear oil should work, but be sure it has the additive for the "powerloc" clutches It won't hurt the non power locs, but the others need it.

The XJ40 rear is hard to pin point any non-standard sounds. From fulcrum bearins, U-joints, hub bearings to the diff itself. If the diff fluid is clean, start listening for bearings. But, then if the unit is a powerloc and doesn't have the friction modifier, that could be the sound you are hearing. Do not use the pink modifier found at most autoparts stores, go to Ford and use their blue stuff. Uses less and does the job well.

Well, forget Eco Lube, they would not honour their quote yesterday, and wanted $60 today. If any 85w 140 gear oil should work, I'll take care of that this weekend when I re-do the output shaft bearings. I called Jag today after Eco Lube said they wanted more money, and the tech. their said I shouldn't need my diff fluid replaced, that my noise is probably the output shaft bearings. I figured that it won't hurt to replace them since the car is sooo old.