Orchidskip,
I'll go through the obvious.
Are your springs mounted under or over the axle? From the pics it looks like over.
Is this the original axle (i.e. an Eaton)? If not you could be losing an inch or two of height from that.
A 4" lift block is pretty normal thing for big 4x4 guys. Any spring shop should be able to make you some new u-bolts that will fit. But this is not a solution to the problem, only a bandaid. But hey, what the heck... it'll probably work great.
Don't know if I believe the cadillac comment though.
