x2 on spindle bushings. My step-son's 91 developed a similar sound, although in his case, more pronounced at low speed when cold. Turned out to be worn (and dry!) spindle bushings. The bushings apparently dried out over time, and then wore out to the point where the excess clearance allowed the rubber in the seal on the back side of the spindle to actually chatter against the sealing surface. Made a very loud grinding-like noise from inside or outside the truck.

As I recall, there isn't an effective way to grease these short of disassembly of the spindles. We replaced the bushings and seals, greased them up good, and solved the problem for at least another 80K miles -- was still okay when we sold the truck.