Did you shim the diffs at all? Are all the bearings good, espeically the ones that support the ring gear/diff itself? I've put 3906/old 3.3-2.5 diffs through he'll with no problems when they are shimmed, even with plastic 3.3 cases. You only have to shim the pinion, but the driveshaft or cup must be attached. Why, any cup or shaft that uses the stock style drive pin helps determine the pinion depth, even some with grub screws do as well if they line up with the hole in the pinion. You can pm me for further explanation or if you'd like the hardly used pair of Integy cases with perfectly shimmed 3.3 gears/bearings I have no use for
I know what everyone says about integy, but I have no complaints with these cases.