I'm having the same problem and I was wondering about doing this. But I have 2 questions How would you keep the slip yoke on the tailshaft? Will it hurt the engine/tranny running it up to 5500 with no load on it?
What I would do is have the car on stands and yoke in the trans put a jack stand or something like it with a flat plate mounted to the stand. Slide the stand in behind the yoke just so its not touching, so the yoke can move some. Run the car to the speed/rpm thats the problem point.At least at this point you could say its there(vib) and know that its a trans/engine issue or no the vib issnt there and that would say that its a d-shaft rear end problem. Running the engine and trans isnt a problem with no load but when you slow it down put it in N and shut the engine off, dont put it in park with the engine running
