Front End Vibration @ Speeds>40mph, M925A2, Found and Resolved

[74M35A2]

I had a vibration issue at speeds over 40mph that would clear up when I switched the front axle on (18,000 miles on odo). Due to this, I immediately started looking at the front driveshaft, and suspected something in the U-joints or splined slip joint. I could move the spline joint some when pushed side-to-side with a forceful shove. I saw a single sentence in the scrap yard forum here that mentioned M939A2's needing the carrier bearing that was present on previous version M939's, but deleted on later versions such as A2's (I'm not sure about A1's), so I purchased and replaced my front prop shaft with a used one which had the carrier bearing on it, and added the carrier bearing support plate. This was all a bolt on deal, as the holes were already drilled into the frame cross member for it. Supposedly the long and heavy driveshaft weight wears the spline joint prematurely. With installation of the replacement (used) front shaft and newly added carrier bearing, the vibration is completely gone, butter smooth all the way up 67mph. The driveshaft weight is now carried by the bearing, and not the spline slip joint. This front prop shaft has one additional U joint on it at the spline slip joint, which is relocated to the front of the shaft vs the rear on the A2 version. I'll update this later with a picture, but if your A2 has a front end vibration that disappears when the front axle drive is initiated, check your splined slip joint for side play, and then consider to back-date to a carrier bearing type.