Unlike many others who offer only the front differential bushings, Maximum Motorsports took the time and effort to analyze the IRS system. The MM Engineering Team determined that all of the differential bushings must be replaced as a complete set. The complete set includes the two front bushings and the essential single rear bushing.
If you only replace the front bushings and leave the original rubber rear bushing in place, you'll create a mismatch in the stiffness of the bushing materials. In other words, the more rigid front bushings will deflect less, while the remaining rubber rear bushing will still deflect as much as ever. Damage from wheel hop will still be possible because the rubber rear bushing will deflect and allow the differential to flop around.
The mismatch in bushing stiffness will increase stress on the aluminum front mounting ears of the differential housing, which can lead to fatigue and stress cracking of the housing.