Does it happen regardless what gear it's in? If so, then it's probably the driveshaft. If it's RPM dependent, then I would say motor, but speed dependent is definitely drivetrain. Our driveshafts our notorious for losing the weights and giving bad vibration problems. Also, do you have aftermarket exhaust? I had some flowmasters on a while ago, and when I did my dumps, the passenger side muffler worked it's way over and butted up against the driveshaft, causing a HORRIBLE vibration between 60 and 70. The solution? I went to SLP, lol. Good luck