I would probably try one in the front. It seems to me like thats where you would see the most benefit from it, and the traction load would be less than the rear. Im by no means an expert on this subject though, my only experience with this kind of diff is from full size cars. A lot of performance oriented awd full size cars run limited slip front diffs with open rears (EVO, mazdaspeed6, WRX, etc). Are you running a center diff or the 2 speed?