The PO sure didn't do you any favor by welding the u-joint. Not sure if you can get enough clearance to get the driveshaft free but try this. Make sue you have the truck blocked on the frame, not by the rear end. Use whatever strapping affair you can devise to pull the rear end rearwards on it's pivots. The rear end may be able to be pulled far enough back to free the driveshaft or at least give you more access.
