The rear end sag is probably due to tired springs. They don't hold the rear up as high as they should when they get old. The differential mount could also be tired.

The hard shifting is most likely vacuum related. Danged near everything, or so it seems, is run off vacuum in these cars. Door locks, engine shut off, EGR, transmission shifting quality, climate control, and various other things depending on the year and model. Do some searching on vacuum leaks. When the system starts leaking the transmission doesn't receive the proper amount of vacuum which causes hard shifting.

You clunking noise under changes in acceleration sounds like a bad flex disc on the drive shaft. Or the center bearing on the drive shaft. The flex discs are what hold the drive shaft to the transmission and differential. The center bearing is what supports the center of the drive shaft. This is an important one to not let go. If one of the flex discs give out your drive shaft will flop around under the car. And at highway speeds you can imagine the damage it will do. Climb under the car and inspect both flex disc. Your looking for cracks or miss-shapen rubber. For the center bearing wiggle the center of the drive shaft back and forth some, it should move like 1/2" to an 1" to one side or the other, but no more than that. Also look at the rubber.

Putting the car into reverse often lifts rear end because of the torque of the engine and transmission wanting to move in the car in that direction. I notice on my wagon when shifting from reverse back into drive the rear will drop back down for the same reason. The clunking isn't a great think, but again pretty normal on these old cars. But I'd have a look at the flex discs and center bearing as well as the differential mount. All of those things could be contributing to the clunking noise.

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