Did you loctite the pinion grub screw? If you did you may have to heat it a bit to melt the loctite. Try a different size hex driver (ie metric or inch equivalent or a torx bit, sometimes you can force one of those into the hex on the grub screw). If its totally stipped and you still can't get it out, take a rotary tool (dremel, die grinder) with a cut off wheel an cut a slot in the set screw so you can remove it with a flat blade screw driver. Hope that helps.