My wife and I just bought a new 2003 TrailBoss 330. After riding it for about 5 hours it tends to shake bad enough in the rear that you can feel it in the steering. I checked it out the best I could and found that the lug nuts were loose on both wheels to the point of unscrewing a few of them by hand. I retorqued them and checked all pressure, but it still has the shake now. It does appear that the rear tires run out of true but I'm not sure that it would make it shake like it does. Any thoughts or comments on what could be causing this would be helpful.
Thanks!

 

Bent rims. Happens to cars too. There is the rare chance that you bent the hubs or axles, but the rims should be the weekest link and go first.