bflooks

a little history...98 4Runner, 5spd, 2.7l, 4x4, 176k miles.

the truck has developed this high pitched squeel/rattle that starts in 3rd gear, at 2100 rpm and again at 2800 rpm. i can not pinpoint the location while driving.
out of fear of a motor issue, i ran a few of the obvious tests...

1) truck in gear accelerating, noise occures
2) truck in gear deceleration, noise occures
3) truck in nuetral after passing the rpms in question, noise occures as motor passes them
4) truck in neutral and engine revved to 2100 rpm while moving, noise occures.
5)truck parked, in neutral and engine revved to 2100rpm, noise does NOT occure.

it appears to be limited to motion, and when in motion, engine rpm's. we put the rear up in the air and attempted to pinpoint the location, and the best we could get was somewhere towards the back. not sure which end, as we were not going to get under the truck during this test and put our ears near moving parts.

fluids are all full.

any suggestions? me and a few of the ttora-ne and ne4roc guys are stumped.