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I asked the question in another thread about changing the rear differential fluid on my 1973. The best thing to do is to pop the cover off and do a full cleaning and inspection. This brings me to now asking, how much trouble is it disassembling the rear suspension that's needed to get the cover off?
Nobody does that to change fluid. The cover is bolted to the crossmember. To get to the bolts you have to drop the crossmember.
You buy a hand operated fluid pump to suck the old fluid out.
While removing the cover would certainly be the best, that is A LOT of work to change the fluid. I don’t think you can remove the cover without dropping the diff.
items that need removed just to access diff: spare tire carrier, exhaust, strut rods, leaf spring, half shafts, driveshaft. Then just about everyone will drop the diff and crossmember as one unit. I have removed diff and left crossmember in the car but it’s not easy. The crossmember bushings are usually stuck and take a combination of a large pry bar, puller and cursing to remove.
summary- suck the fluid out and replace thru fill hole.
If you have excessive noise, or if the fluid you extract is full of metal, pop the cover and take a look at the wear. Buy a fluid pump for an an automotive store to suck out the old, and push in the new. I use gl-5 80w90, buy the limited slip additive from AC Delco, i pick mine up at a local dealer. Buy 2 bottles, 1 usually works but a lot of people on here use 2 to keep the clutches quiet.