Probably time to get this car up on a hoist and start checking things out. Its not going to fix itself and since symptoms are changing could be the damage is increasing since original clunking or whatever. Certainly cheaper to diagnose or have diagnosed before car leaves you stranded and perhaps in hands of total strangers to get it back on the road. You could jack up one rear wheel at a time and see if it moves by grasping top and bottom. Pushing the top and pulling the bottom of wheel and reversing the procedure. In fact just grab the top of each tire and push and pull hard with car sitting on ground should suffice. Just compare to other side as both should have equal play if any. (possible rear bearing failure) If car passes that next proceed to compare cv joints on one side with the other by turning tires back and forth. check for abnormal clunks, play, etc again it is by comparison of one side to another that counts. Also grab 1/2 shaft and push up and down aggressively again comparing one side with other. Above tests may reveal nothing but there is also a chance they will. Plus as an added bonus you might repair it for less than the towing charge if it goes down on the road. Make sure it is also not just a loose wheel check your wheel bolts. While you have car securly jacked and blocked up or on hoist it is quite easy to check your centre driveshaft bearing carrier and rubber universal disks and make sure everything in the back end is bolted down. Examine your rubber bushings that support rear suspension to see if they have wore out as well. In otherwords have a good look around. Since people on this site can not actually hear the problem firsthand and have no knowledge of your experience level i assume you have checked that all the rubber muffler hangers are in place. But the up and down gyrations are raising a little concern in my mind and especially as they do not appear to change with speed. (lets out tire problems like broken belts etc).