I think I have this one fixed. When on a rough stretch of highway when hitting a 'hollow' and the truck's suspension squat's I was having a thrumming sound like a u-joint going out. What it turned out to be (I think) was when the driveshaft changed to a shallow angle- the grease nipple on the slip joint was hitting one sagging heater hose to the rear heater and had over the years worn a groove. Could have been ugly on some trail if left alone. Tied both up higher with zip ties.