I have a 2000 GMC Sierra 1500, RWD. Everything I go on the highway driving 50+ mph, the steering wheel will have a slight shake or vibration. I have checked the suspension by jacking it up and seeing if there’s any play in the wheels and there is none. The truck has a brand new steering rack, tire rods and an alignment. I also got all the wheels and tires rotated and rebalance but the shake is still there. Any ideas on what it could be?