Well, unfortunately, no smoking gun.
Rear suspension is tight all around, nothing loose.
Spun the drive shaft and the giubo looked new all around.
I couldn't see the center bushing without pulling the exhaust & heat shields, so I have no idea how it looks.
After I pulled it off the stands, I opened the hoood and blocked the front tire (parking brake is not working). I then let the clutch out in first, rev'ing the engine to put a load on it while I watched the engine through the hood. No excessive engine movement telling me that the engine mounts are OK. I'm somewhat tempter to take the hood off and watch the engine when it bucks to be sure.
While under the car I did notice two things that I'm not sure how normal they are:
- If I grabbed the transmission and pushed side to side, there was a decent amount of movement, maybe 1/4" - 3/8" each way. The mounts looked good, but were pretty soft.
- If I grabbed the half shafts amd pushed side to side, each moved a fair bit as well, the driver's side a bit more than the passenger. Probably 3/8" and 1/4" respectively. No torn boots or axial play.
- Ahead of the transmission cross member was a lighter duty bracket with two torn rubber mounts on it. Obviously, whatever had been there was gone. Some kind of splash sheild maybe?
I'm not sure what to think at this point. I guess I'm leaning toward u-joints and maybe the center bearing, but that's just a guess. I may have to concede and take it to a shop for a diagnosis.