This could be the a couple of components from the transmission, and everything from the front propshaft to the rear diff ("final drive") is worth checking.
My money would be one of the three mounts for the rear diff, particularly the front one. This can wear and allow too much play in the rear propshaft. In extreme cases, this can cause the rear UJ on the rear propshaft to grind at the fuel tank!
The Haines manual has a good description of the transmission so Irecommend a read of that with a cuppa and then a trip to a friend with a good set of axle stands or an inspection pit.
Cheers
Blippie