I am in the process of changing out the fluids in my 90 since I don't know what has been done when. The car has 50K and the book is calling for the usual thing to be changed. Tranny fluid and radiator flush. I didn't see anything about changing the rear end grease. I sat next to a mechanic friend at the game the other night and he said it's like a 6 hour job for them to do it. I'm like, there is no way! So once again I differ to the wisdom of the guru's. Can you help me out with that? Basic stuff but I'm learning. Thanks Ultra (and how do you write the stuff at the bottom of each thread? Like mods on your cars and stuff.)

The C4 diff does NOT have a drain plug as deliverd from GM....

You have 4 options:
1. Split the batwing and housing. (The 6hr option)
2. Use a "suction type" drain out the fill plug.
3. Drill and tap a drain plug.
4. Don't change it. Add fluid as nesescary....

The stuff at the bottom is called your "Signature". It is located in the user options page under "Edit Account".
Will

Unless your differential has been modified from stock, it does not have a drain plug. I would suggest buying a suction gun from your favorite auto parts store and use it to suck out all of the old fluid (not grease). Clean the gun out and use it to suck the fluid from bottles of new fluid and refill until the level is just below the plug when the car is level.

The type/brand of fluid is debatable. Personally, I would steer clear of synthetics unless you race the car. The chance of creating leaks is fairly high and they are a pain to fix. I would just use a decent brand 70/90 or 80/90 gear oil and call it a day.

Be sure to add 2 bottles of posi additive. This is available from the dealer or there are some gear oils with a comparable additive already mixed in.

+1 on using the hand pump. I used synthetic on mine. Don't forget the GM friction modifier (2 bottles). If you search under my name I have a long thread from when I did my rear end rebuild and the info is in there.

Matthew