My 1995 G-20 van, V-6, 4.3 liter, automatic transmission makes a "thunk" when I

Possible engine/transmission mounts are worn .. Usually when a universal joint gets that loose or worn it would be vibrating or popping/squeaking while driving... Still good idea to have em checked out - along with rear gear lash - (ring gear/pinion). Drivetrain noises should still be present while driving down highway... Not so much with mounts... Have 'mechanic' drive it.