Generallly you can jack it up and grab it near the joints and just try to wiggle it....or, like someone suggested, have someone get in the truck and watch the driveshaft as its put into gear, then have them put it in reverse and see if it is moving around any. Just from the conversation I wouldn't go messing with the pinion angle just yet. To many other things can cause what your talking about.....I've even seen the little weights come off a drive shaft causing a little vibration.
Also, do not rule out tires, rims ect.
Do a little checking and let us know what you find out.
