Sounds like the bushings in the differential are worn out. Maybe bearings on the axle too.. Could be rim... or tire... Start with swapping tires from one side to the other. If it does the same thing on the opposite side I would say tire and or rim. If you grab the axle where it goes into the differential and it is loose then the bushings are probably toast too and then if you keep riding the gears will wear and be junk.... I bought a differential off YELLOWHAND and swapped my axles with the new differential and gears from the one from him. LIKE NEW again.... Randy