Folks my f-350 SRW is on the third one since June of 2009. I chalk the first up to bad install. It failed in two months, the second lasted from aug '09 thru nov '11.

For some reason, I think my problem is made worse when I tow my fifth wheel. At first I thought that hitting dips and buckles were causing the trailer to squat the bed ( I do have air bags installed) and that squatting might cause more axial displacement. My line of thinking was that maybe lack of proper lubricant in the splines may have caused the shaft to move because of resistance in the splines.

I still do not know what is causing the problem, but after a 10 hour pull, I had a failure, and two years later another failure after a 12 hour pull.