Have you checked your propshaft? Try running it with the manual hubs in lock position, the tranny in 2WD and the front prop removed.
If the noise is still there I also would recommend checking the brass bushings.


Black '90 4R

SAS with LC80 coils and LC70 high pinion axle
LC80 rear axle, coils and locker
4.88 gears
2JZ-GE+turbo+intercooler