I would first guess that the rear control arms may have been tightened while one side was loaded and the other side was not. Your mechanic probably did it correctly and I am guessing that you have already checked this but "just in case" it has not been addressed yet:
Start off with all of the control arm bolts VERY loose. Place car on ground or put on hoist. The rearend needs to be completely loaded no matter how you do it (not free floating). Bounce car up/down several times. Now tighten all control arm bolts.
You could help eliminate any possible bushing binding by loosening ALL control arm bushings (front and rear) and bounce on all corners, jack each corner off the ground, etc, then put back on ground. If it sets level with the bolts loose then it's the bushings binding. If not then it's probably a bad spring or bent frame. A 4 wheel alignment can tell you if the frame is bent and how much.
If you have done any front end work it will cause the same affect with binding bushings. It's better that all bolts are tightened at the same time while suspension is loaded. Not one side at a time.
Which springs did you replace? If you only replaced 2 then it could be one of the "other" 2 that are sagging.
Check to make sure someone didn't install the wrong length control arms on one side in the rear. If one is shorter it would cause it to lean.
Sorry if this duplicates anything you've tried already..
87GN, 12.05 @ 111, SIXTY FOOT 1.60-LAUNCH 15#, BOOST 25-26 LBS (YEP OVERKILL FOR COMBO), BONE STOCK LONG BLOCK, STOCK I.C. (BELL MOUTHED), STOCK TRANS/CONV, TE44, 009'S, RED'S 107 CHIP. HOME MADE 3" DOWN PIPE. RUNS CONSISTANT 12.10'S-12.11'S.
anyone ever seen a gm car that dont lean?
ive been a gm fan all my life and have owned way too many and never had one that was exactlly level.
try measering fenderlip to ground on each side front and rear then do the same from frame to ground in about the same locations.
you may be stunned
85 ttype going under the knife soon for 87 transplant but wont be giving up on the hotair set up it will be in another car. so far hot air been [email protected] with many stock parst still intact including stock 85 ecm and chip,stock turbo self ported,stock ex. headers port matched,stock injectors in need of replacement,self modded intake,home brewed port work on stock heads,stock valves never had a valve job with 183,*** on the clock,sealed power replacement cam,atr 21/2 downpipe,stock up pipe,stock fuel pump NOT hotwired,kb billet reg,and the best thing ive done to it yet is..........wrapped fuel rail with headder wrap that made a big inprovment in restarting and drivability.ok enough said