I would simply ask the dealer if they will be willing to accept the liability of the welding (and they will be liable if it ever went to court). The potential damage from the drive shaft breaking loose or being out of balance and ruining the transmission should make them re-think the deal and insist that the third party who welded it make it right. I wouldn't risk thousands of dollars of legal fees and a ruined reputation over $175.00 worth of work that obviously looks shoddy from the get-go. And neither will they.