check the compounds of the tires.
for instance running a m2 compound on one side and a m3 compound on the other will cause this..
also check the two screws on each side that hold the inner arm mounts,, one loose and with play will cause a variable toe in thing and can cause the truck to side track.
have you recently changed out the rear hub carriers or maybe just one? if so maybe there is a mismatched one on there causing one sides toe in to be different than the other.
still check the chassis for twisting (tweak) as well..