I'm w/nailhead on this one,try and run an actual bushing in the upper ears of the differential.rubber if possible.it'll help isolate noise/vibration/harmonics better than anything else.that being said I'm running umi roto jointed upper and global west lowers.spherical bushings on the frame side of things and derlin bushings on the rear end side.these type of bushings do a great job of keeping everything in line and help with travel as the design in the rear of these cars is constantly in bind.the Roto joint/spherical bearing is prone to transmit a little through the chassis compared to a rubber bushing but the trade off is worth it.