Are one of the dogbones(front driveshafts to the wheels) bent? Also hold the truck off of the ground and spin 1 front tire. The other tire should spin freely in the opposite direction. If not you may have a bad front differential. if that's OK then keep 1 rear and 1 front tire on the ground and try it. The 2 tires in the air should spin free. Check you brake to be sure it's not dragging. When you tightened that grub screw down was it tightening to the flat spot on the shaft???