define problems? Noise/clunking/breaking?
I know if the gears are not put back in correctly or all the right parts are not replaced correctly there will be issues.
Ask them if they:
checked the back lash, the pinion depth, the crush sleeve was set right, the wear pattern was set right, etc if they just poped them in... there is your problem
also if one of those little nuts on the ring gear comes off... its game over