Possible Diff Problem?

Hi, all. I have a 96 RMW purchased last fall from a gentleman in Charleston. It has the tow package, is more or less stock, and is an awesome trade-up from my 2010 Cadillac CTS Sport Wagon. It had 90k on it when I bought it and is now at 98k. I think I may have a rear-end problem, and here is why:

About six months ago, I noticed a squeal or moan from the rear end when I would make a tight turn in or out of a parking spot. Driving in a tight circle, either forward or reverse, would elicit the same moan, with an occasional "jerk" felt in the seat, sometimes accompanied by a tire chirp. These tests were accomplished at or just above idle. To my relatively untrained ears, it sounded like maybe clutches in the rear end were grabbing. So, I removed the cover and inspected what I could. There were no big chunks in there, and just some normal swarf on the magnet, so I serviced it up, adding the GM Posi additive as per spec. The moaning and jerking went away immediately.

Fast forward to today, where I noticed a vibration in the seat, intermittently starting at about 40MPH. There is no moaning like before, but slow turns in the parking lot cause jerking that seems more violent than before, although it didn't chirp the tires. The most worrisome thing was during a brief highway run, at about 70MPH, there were a few brief periods of rapid deceleration that did not appear to be caused by engine malfunction. It also appeared to require more throttle input to maintain speed.

So, I'm thinking the diff is toast, but I'm a relative novice at geartrain stuff, although I'm a pretty accomplished mech in most other areas. Any thoughts on these symptoms? Any troubleshooting advice? Help is appreciated.

Could be the axle(s) and/or axle bearing(s). The first place to look is the passenger's side. the passenger's side is the first to wear. Check for grease leaking out of the seal. You should also check the driver's side for grease leaks. If the axle is moving around, it could be pushing the drum against the brake shoes. If you need to replace the bearing, you also need to replace the axle, because the inner race is the axle itself. If the driver's side is still good, you can check for junkyard driver's side axles for use on the passenger's side. Wagon axles are unique to the wagons. If you get a good junk yard axle, you still need a new bearing and seal. There should be no groove in the axle at the bearing. If it is not the axle or bearing, it could also be the transmission.

When you refilled the rear, and added the posi additive, did you use conventional or synthetic fluid?

I used conventional fluid.

pull the diff plug again dip your pinky in the oil and see if it smells burnt,.

Rapid deceleration sounds interesting and dramatic,. depending on what scale we are talking here tho,.

At 70mph, under steady-state throttle, it dropped maybe 5MPH in a second.