Ridiculously late follow up. The rear wheel bearings appear to be fine. I changed the rearmost u-joint and it cured the growling noise. I do still have a strange wobbling sound coming from the driveline but it comes and goes, its not steady like the growling sound was. I'm going to change that center support bearing and go from there. I just need to get around to ordering it.

97 T100 4WD SR5 - 231K