I had the same issue with vibration at around 20 MPH and then again around 40 MPH. I swapped from a Super ATV to a Sandcraft Gen 3 carrier bearing, which helped a little bit. The sandcraft has bushings where it mounts to the frame to absorb some vibration. I greased all the drive line zerks, upgraded to new HD rear axles, balanced my wheels, still had bad vibration.
After all that I knew it had to be the driveline. I looked at SuperATV's website and saw they came out with a new C series drive line that use CV joints instead of U joints. I also noticed their bounty series looks to be discontinued since they are all out of stock.
I emailed SuperATV about my issue and they agreed to swap my bounty series drive line for the new C series for a small fee, since I've had my drivelines for awhile. I also went back to a billet aluminum SuperATV carrier bearing to make sure the whole setup was supported/warrantied by SuperATV. SuperATV was great to help me out.
I got the whole setup installed and finally test drove it today... NIGHT AND DAY DIFFERENCE. It's so smooth and quite! I can't tell you how relieved and happy I finally am. After spending all that money and to have those bad vibrations was driving me crazy.
As an extra measure I did apply some sound/vibration deadening (generic DynaMat) material on the bottom of the frame near the carrier bearing that got sandwiched by my full skid plate. I was just hoping to minimize vibration traveling through the frame if there was any. The SuperATV carrier seems tough and was smooth with this setup as well.
From my experience they figured it out with this C series, too bad it was a PITA for me with their old product. To their credit they worked it out for me, which is a sign of a good company IMO. I just wish I got sponsored or something....
... I need red high clearance A arms
Check out the SuperATV C series drivelines.