That does not sound good.....perches that bow down will pull more arch out of the pack. The perch surface must be flat. I had it happen once on the front on my 84 Yota pick-up....the solution was to weld a thick length of flat steel on-top of the perch....maybe 2"x6" flat-stock or similar. I had mine 9 inches long.....kind of had an anti-rap effect. A flat-stock brace at each end is a must.

Another item that will cause your condition.....and this just happened to me 4 months ago.....I bet this never happened to anybody on this board...it drove me nuts and caused mucho aggrivation .....is when the perches were welded to the housing that they were not "even" or parallel with each other.

If that happens, then when the u-bolts are cranked down, it pulls one pack differently than the other and one pack gets pulled down making it lean bad. 2 inches in my case.

Loosen the u-bolts all the way and bounce on the rear packs a little and see if it levels out. If the rear levels out, then one of the conditions above has your name on it.