When coming to a stop and releasing the break there is a clunk coming from the back of the truck.

I suspect you have a two-piece drive shaft and where they join in the middle is a splined slip shaft that allows it to change length as the rear suspension moves up and down. Most of that style have a grease fitting on the side to keep the splines lubricated so they can slide freely. When they bind, braking forces might put stress on it, then when the truck stops, those forces are gone and the stuck splines pop free and slide. That's what I'd look for on any other truck although that is not a common noise. GM is always experimenting with something new or different so there could be something else going on.

Unless someone tells me differently, my guess is there was nothing special about the grease your mechanic used. He probably listed the part number because that sounds better than "lubed drive shaft". If you do your own oil changes, grease fittings are common things that get missed. Even at the speedy lube places they often don't look in back for grease fittings, so perhaps it has just been neglected.