Had transmission rebuilt a few months ago,so I know the problem doesn't lie there,nor the u-joints.When accelerating first to second gear then releasing the gas pedal it seems as though the truck still wants to go,then a "popping" noise. It has done this since the truck was purchased and after the new transmission. I applied royal purple to rear end,after a thorough cleaning with brake cleaner. I ran truck about 30 miles.It remained parked.By the next week it was if the "clunking" had never been there. It has since dropped to about 15 degrees and came back around 85 in one week. Since then it has come back worse than ever.What would be the verdict?Truck has around 250k miles on her. Should also note,metal shavings couldn't be found in the housing.

 

Have you head the ball-joints checked?