Deep, Bassy Rumble Only at Very Low Speed

[GraniteCow]

I’m getting a very deep rumble at low speeds (low speed, not RPM) that sounds like a really mean subwoofer blew out. All new wheel bearings, newish motor mounts, no front driveshaft. Sound goes away after about 5mph and is independent of RPM. 4.0 5spd manual 4wd 1996. Any ideas what it might be?