Okay, lots of info to give you here. Almost all of your answers can be found by searching, but I will help out a bit. It sounds like your rear differential is having some problems. This may be that it just needs some shimming, but its probably a good idea to get a new/good used one and just replace it. If your problem gets any worse I highly recommend not driving the car, as the bad diff or drivetrain part can lock up (this is VERY dangerous) and is known to happen on these cars at 50-80 mph (ask me how I know...).
As for what is interchangeable, 1G DSM with a manual transmission has the same ratios and is therefore interchangeable. (this should be confirmed by searching or a knowledgeable member)
Options:
-3 bolt open diff (Stock on VR4, direct bolt on)
-3 bolt viscous lsd (Needs the lsd axles, otherwise direct bolt on with axle replacement)
-4 bolt viscous lsd (Needs the 4 bolt axles and cups)
All of these will work, however, if you want to use the 3 bolt lsd or 4 bolt lsd you need the axles as well because they are all different from each other. For the four bolt you will also need the cups.
If its not the rear diff, and it may not be, hopefully someone can help you out. I'm sure we will get other members in here to confirm my info and hopefully give some insight on how to check to ensure it is your rear diff that is the problem. Thanks and good luck.
-shamus