i had this at one stage and discovered that one of the output shafts had pulled its self out of where the c-clip holds it into the diff itself. get under there with the rear wheels off the ground a see if you can pull the output shaft out of the diff a bit. if you put in a different subframe as you say it might be causing the arms to pull on the output shafts. take the diff out put in a new c-clip and tap the output shaft back in to its place and bolt her up. is the subframe off another s15 or something els. mine did it after putting in the driftworks arms, guess they were just set to long and pulling on the shafts.