steering vibration

steering wheel vibrates between 30-40 mph during acceleration only. Vibration stops when cars shifts to passing gear. Car is a 2003 mustang V6 automatic. All tires and wheels were exchanged with new ones.NO CHANGE. Differential pinion gears changed(recommended by Ford) NO CHANGE.All suspension parts checked OK. Drive Shaft checked for balance OK. All work done by Ford Dealership. Engine and trannie diagnosed no problems found.

HELP!!!!!

did you get an alignment when you got the new tires? that is always my first thought when i feel a vibration.