3rd. is a "reduction", with the power being transmitted through the cluster gear, then to the output shaft. 4th. is direct drive, straight through the trans. It's unlikely that anything in the transmission, except possibly the output shaft bearing, could be involved. That one is under a cover plate, just ahead of the flange that connects to the driveshaft.

If you have a 2-piece driveline, the center carrier bearing could be making the noise, or the pinion bearings in the rear axle could transmit sounds forward, giving the impression the noise is from the transmission. Have someone ride in the bed on a test drive, and see if they can get a different perspective on the noise. Check the universal joints, too!
Jerry

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