When checking the rear driveshaft it's best to jack up the rear end and put it in neutral. This will take any tension off the shaft allowing you to turn it freely. I've had bad u-joints that felt fine until I jacked up the rear end to check. Also check to make sure the balancing weight hasn't gotten knocked off the shaft. Make sure your tire lugs are secure. I once thought I had a bad u-joint but ended up being loose lugs on my alloy rims and the tire was wobbling. Felt stupid on that one. Oh, chock your wheels when jacking. The only other thing I can think of is a lost wheel balancing weight, mud stuck to your rim or a bad wheel bearing.