Need a couple other opinions here guys. My 89 F350 PSD was making an aweful vibration almost grinding like noise when hubs are locked engaged in 4wd. It happens around 20 mph and it only really noticeable when I accelerate. So I got underneath and looked for anything I could think of, I noticed on the front drive shaft that the double-cardan joint or CV as some like to say was very loose, I could wobble it around by hand and it made a clicking noise each time I tried. Im guessing this is the problem, but am not quite sure how much play is really suppose to be there. I got under a 2008 F250 and did the same to it and notice the double-cardan was rather firm. Anyone have experience with this?