1994 GT issues

I have/had a '94 GT. A couple years ago the rear end exploded and I just recently got it back together. In the rear are new FR 3.73 gears, clutch kit, all new seals and bearings. The car had been painfully neglected over it's life so I put on new rear rotors, pads, shocks, upper/lower tubular control arms and every bushing in the rear is now polyurethane. The only original parts left are the coil springs and quad shocks. I took it around the block a few times then sold it. The new owner has been driving the heck out of it the last two weeks and it's developed a problem. When you accelerate hard from a roll in 2nd gear on the car pulls very hard to the left. It hasn't been doing this til a couple days ago. I drove it today and I'd swear it had a broke axle but it doesn't. I thought maybe a problem in the LS clutch packs but it spins both tires and doing tight figure 8s it doesn't drag a tire or make any strange noises. All the bolts are still torqued to spec and all the bushings are still in place. Coil springs are still seated. Even with a 4 ft rock bar I can't make the rear move side to side. I know the guy I sold it to and even if it were a stranger I wouldn't feel right about not helping him out but I just don't know what else to look at. All the tire pressures are good and I swapped the rear tires left to right with no change. I'm stumped. The only thing I can come up with is maybe the quad shocks. The control arm kit said I didn't need to reinstall the quad shocks but they didn't say I needed to leave them off. Could this be causing some sort of binding when the car tries to squat down? On a side note, and the new owner knows the history of this car as we both new it and the guy who did all the work on it around 2010, this car has been wrecked before. The owner before me got happy with the nitrous one night and put it 10ft down a bank into a creek. We thought it was only cosmetic. It damaged both bumper covers and the driver's side sideskirts along with the rocker panel. After that it sat until I bought it. I drove the car for nearly a year daily and never noticed any issues but it still had the ruined 25 year old bushings and shocks. Maybe with the rear tightened up it's reflecting an issue in the front of the car? Seems like if the subframe were bent or twisted it would pull all the time though and it drives straight as an arrow under normal driving and doesn't chew tires up or anything. I'm at a loss here. Any input? I want the new guy to be happy and I really don't want to give him his 5k back.
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