While in neutral, rotating driveshaft back and forth I hear a clanking in the tail shaft section.
Something else I just discovered, while rotating the passenger side wheel back and forth I get some play before the drive will turn and hear gears meshing?
That is what Sherlock Holmes, Lieutenant Kenda and Columbo would (likely) call a (serious) clue(s). The clanking sounds real serious & I think that is what you are hearing when it shifts, if you can hear it now with what you are doing just imagine what is happening when the load is released on a shift. & thirdly the ujoints should move SMOOTHLY in there plane of travel (both ways in front yoke) back & forth.
