the"RoadRunner"

hey, i have a question that hopefully some of you can help answer. my 91 with a 3.0 and a 5spd has been making some weird noises on acceleration. the noise sounds very metallic and like something metal is rolling or clattering around in a metal container. the drivetrain has 220,000 miles on it, and the noise is very intermentant. it mostly happens around 3,000 to 3,500 rpm in
3rd. but, does not always happen. i hear that this could be the transfer case?! what do you guys know about this.

all help is greatly appreciated.