Yes, check all the drive line for play. I usually find that vibrations when letting off the gas are due to something being loose. Common source is a loose pinion or t-case flange or a slip yoke that is out of grease or pulled out to far that it is loose. Could also be u-joints. Basically the checking is all the same, climb under the truck and then just grab each part of the drive shaft and give it a tug and feel for any play up-down or side-side. You want to have it out of gear so things are free to move. And you an also unbolt the shaft at either end and feel the u-joint motion by hand as some times the joints get loose as they wear but they may also feel too tight and bind up as they rotate.