You have 5 trucks, so you are not a rookie and I'm not trying to tell you what to do, just my 2 cents. We have all been fooled by vibrations, the shop could be correct (too short will cause wobble) but check engagement per advice above. Also if you have any number of other causes, like tires, or motor mounts or loose steering components or bad Ujoints, they may not show up on static test.
As far as pinion angle, we don't know if you have lowered or jacked it up or other mods which could change pinion angle. As far as how you would know: measure with angle finder.(3-4 deg at both ends). How to correct: post/decide after measure.
The main issue with Forum help (not directed at you) is that we don't know when it started, what was done recently, mods, changes, etc? Another person had vibration and found it to be the tranny output spline/bushing. I think we all decided that at that bushing there should be very, very little radial movement (sideways slop). The sliding in and out is axial movement.
