alum inserts on the rear diffs ? have u ever shimmed the rear diff ?....remove the rear end and make sure thats where its coming from.....check with the case open to see whats up.....and take a Gooood look at the teeth of the ring gear....i had one Twist and make the rear end all crunchy when i got my T used.....but once I personally rebuilt it with a new R+P and shimmed it to be Silky smooth it was alll good.