Crack in rear seat area

Welding back up should solve your problem. One thing you should do first is drill a small hole at each end of the crack prior to welding. Doing this will stop the crack from picking up where it left off if your welds possibly fail. Be sure to weld the holes you drilled as well.

Jim
 
crawl under the car, and check to see if: a) pinion snubber is still in place
b) floor is damaged from bad u joint sometime in the past (the ends of the crack look a little distorted, like the universal was hitting the floor)
my floor is beat up, too... from a missing snubber