The bushing in the rear of the 06 that is the culprit, is the upper control arm on the rear, upper, outer bushing. There is a guy over on predatorowners.com, "drich" he has made a few sets. I bought some from him and they really tighten up the rear end. They seem to be lasting quite well also. If you update to the needle bearings for that bushing you will also need to update the outter bearing carrier, I don't think it is a direct replacement. You could probably machine out the upper area to make it work.
06 Outlaw 500, B.O.D.3/2/06, Muzzy slip on, Dynojet Kit (175 main, 3rd needle clip, elevation 3500ft), Airbox lid off, Protaper Contours, AC Blackline Propegs, Spidergrips, Powermadd, ITP Holeshot HD, PRM Baja grab bar, DynaFS CDI, Magura Hydraulic Clutch.