have you confirmed that it is NOT stuck in 4wd? dont assume that because the indicator light isnt on, or that the 4WD shifter is not in it means its out of 4. take it to a spot with lower traction, snow covered, grassy or muddy area and feel it out, forward and reverse to make sure the front wheels arent pulling. that is something you would want to confirm A.S.A.P.
sounds strange that it would work out far enough to cause that kind of action without feeling it want to pull to either side at higher speeds.

also, who replaced the parts, and did the alignment? Should it not be a 4WD problem I would have taken it back to the place that did the work if it started after only 500 miles after work was done.