It sounds like you did a number on the rear axle with that 4"block. Be happy you didn't eat a R&P...
I have 4" Alcans on my 4Runner along with a Marlin Ultimate and 5.29's. I haven't busted any u-bolts but I did bend my rear axle housing under the right spring pad. The front of the pad had dented the housing in and the weld on the rear edge of the pad had cracked allowing the spring pad to rotate ever so slightly forward. Having a 223:1 crawl ratio helped this I'm sure. I beefed up the area around the axle pads on my new housing so maybe I've taken care of that problem.
