You changed the caster angle by removing the blocks, this also moves the axle center line. It takes very little center line movement to change the steering gear center line. Make sure the space blocks you took out don't have offset hole / pin locations.
Likely you have 2 degree wedges, might need to jump up to 4 degree wedges. The steering should come back to center and the feel will be more solid. I have been down this road with my truck, I had 3/4" blocks and removing them did just as your is doing now.
Any time you raise the rear or lower the front, the caster changes, less caster angle to looser the truck drives. Combine that with an off center gear box, it starts to get spooky at highway speeds.
Did the truck come with the blocks from the factory?
