shims. they are pretty stout. been a lot of them in the 9's. I'm putting 9 inch ends on this so I don't have to run c clip eliminators, and im also gonna run aftermarket axles. I figured the money saved on the brakes, gears and lsd, was well worth it though for now. down the road I may go 8 3/4 or 9 inch. hopefully these ford parts don't get me banned from this forum. lol