Guys, I fought this and pulled my hair out. I feel for you! I am sure it could be a number of things, but I have run across this vibration a couple of times.
Turned out for me, my drive shaft was slightly out of balance from the factory. 75 hours later, it ended up wearing the spline connection out and ruining a couple ujoints. I continued to pull my hair out with the best way to fix the pile of junk until I just broke down and ordered the complete upgraded driveline from Sandcraft. It came with carrier bearing, front and rear shaft. This RZR is smoother than it ever dreamed of being. I am a firm believer that you can't go wrong upgrading that driveline. Yes you can "rip it" but, it also saves wear and tear on front diff and transmission output shaft. I think it is pathetic that Polaris can't put a high quality driveline in these things from the factory.
Another RZR I know actually bent/tweaked his shaft while riding. It was not bad, but it developed a vibration from then on. Same fix.
I have also heard great things about the SATV drivelines, but also a couple bad reviews with balance from the factory. I was sold on the other one because I talked to the owner and it is made in the USA. He said ujoints can be replaced, and if the spline ever got loose he would fix it no problem.