You're learning about shift kits the hard way. If you simply shift up, never down, a shift kit is OK, I guess, but on several of the downshifts possible during normal driving, the shift timing is such that you're in two gears at once, momentarily. That locks up the rear end for a fraction of a second, and either breaks the spider gear pivot shaft, or wears out the carrier housing. I've seen carriers with the pivot shaft hole worn into a slot 1/4" long as a direct result of a shift kit being installed. There are better ways to beef up a transmission that don't involve screwing around with shift timing, but I'm sure all the self-appointed experts here will disagree with me.
Jerry
