A friend of mine has an '04 Chevy Colorado 3.5L 5 cylinder 5 speed 4x4 with 185k miles.
He is getting a fairly mild vibration that you can feel through the gas pedal and floor boards. It doesn't sound very loud at all but it is noticeable to the ear.
It is intermittent and not constant. It happens while in gear but coasting down hill the most often. It happens at speeds above 50-55 mph while in gear, in neutral, clutch in or clutch out, and at any engine rpm while in 2wd tc mode.
Any ideas? I was thinking maybe just putting some new diff, tc, and tranny fluids may help out.