I've had this on my 06 Dmax forever. Depending on throttle/load it would happen anywhere from 45-52ish mph. Always when running empty. I researched it at one point after changing various mounts/ujoints/road force tire balancing etc and it seems to be a fairly well-known issue with these trucks...
TH400 with a busted tail housing. Trans had good line pressures and shifted fine when it had enough fluid to build pressure. I picked up an HD case and considered swapping internals myself but I'd screw it up and it was a motherfucker to get out . So I figure it'd be a better play to throw a few...