This one me stumped.Starting to get a rattle sound in the rear section that is hit and miss.  High and low range work fine. I change the oil in it every four truck oil changes. Dropped  the oil today and no metal in it. Pulled the side covers and can't see anything wrong. Pulled the rear yoke and seal hosing off the back and that bearing is fine.

Mine just started to rattle but only when it gets heated up. On the way home usually.

If your talking about the rear of your Transmission, mine was vibrating on my old Cat Eaton trani. It turned out to be a loose nut on the Yoke. The big nut had come loose. Lay under it and push up see if there is any play. I know you said you already took it off, so its probably not the nut, but just a thought..

Yeh I checked that. The mack trans has a different setup on the output shaft. I got it nailed down to a worn gear in the HI-LO section . Getting about a 1/2 of play when in turn the rear yoke. 580,000 on the odmeter with the oringinal trans and clutch. I think I am going to tell the boss to let me order up a Mack re-con trans and clutch pack from wiegand mack and give me a day in the shop to change things out.