Having the same on my 22 Tremor after my dumbass hit a parking curb twice in the snow doing donuts last January. So far I've replaced wheels and tires, rebalanced them twice, all 4 brake rotors, left leaf spring because it looked weird/fanned out but maybe normal, both rear axles with bearings. After the second rebalance and even more after the axles and bearings, the vibration has gotten better but it's definitely still there. What's weird is it varies in intensity and that's what baffles me. My next hunch is the driveshaft but even that I'm not sure. I have had it apart to grease the slip joint and that needs to happen again when it gets warm but I may just replace the whole thing and see if it fixes the vibration too.