I have a 04 Bruin 4x4 and it started vibrating of 15mph, the more gas you give it the more it vibrates.

I also thought maybe its the u-joint going to swing-arm.

 

i dont think it be the belt bore likey the u joints for the shaft. hth

 

are they hard to replace?

 

Could be either.
To check U-joint. Make sure there is no pressure on it, then grab both shafts or on the joints and check for movement, there should be none.
If there are pieces missing out of belt it will viberate, also of there are flat spots where it rides in the sheaves it will viberate. Best way to check is to pull clutch cover off & remove belt. Can lay it on a flat surface & check for flat spots. Inspect for any deformation, cords etc.

 

well ill give you a update when i check it out. Thanks