You mention it's on the passenger side, and that the rear end is a Dana 30. I assume that Dana didn't produce OE read ends for Volvos. I'm thinking a heavier duty differential/rear end (perhaps shifting the driveshaft sideways a bit) combined with a larger diameter driveshaft is the cause. Have you considered a smaller, custom made driveshaft? A little shift combined with a driveshaft about one inch oversize . . . I can easily imagine this collision happening against the tapered (passenger) side of the tunnel.