Why, When in 4WD Does Vehicle Drift to the Left?
 

I was driving down the road about about 20 mph and wanted to check to make sure my 4WD worked and put my Jimmy in 4WD and once I did this the Jimmy drifted noticeably to the left. When in 2WD the vehicle drives straight with hands off the steering wheel.

What could cause this problem?

Vehicle: 2000 GMC Jimmy, 173,000 miles. 4 speed automatic transmission w/overdrive (rebuilt transmission about 5,000 miles ago.) 4 Door, 4WD, 4.3 Vortec V6 engine.


Thanks!