Another simple way to test for areas that might leak is to do as for door alignment. Cut newspaper into a bunch of roughly 2.5" wide strips. Lay them across the seal (not overlapping), close the tailgate/door, then go around and tug on the tails of the newspaper strips to see how easily they come out. Work your way around the opening in sections. Potential leak areas will be where the strips easily slide out. You could try confirming how bad it is with water, either just shooting a jet of water or laying the paper strips sideways just inside the seal. If there's a whole section of the tailgate like that, you would want to try realigning the tailgate, playing with the catch and latch, side bumpers (I'm not sure how adjustable those are) and ultimately the hinges (removing the roof trim panel for access, which as I recall requires removing the side trim first). I've never needed to realign the tailgates in my 940 wagons.