Help with strange grind/rattle in the driveline

Redoing the shocks is on the list because after shackles, they will need to be relocated anyways. I've watched that video and shaken the driveshaft at the 3rd and there's no play. Factory gears, u joints are all tight, can't get fast enough in reverse. Seems to only happen at moderate to high speed when coasting or slightly on the throttle. The diff fluid is next on the list to be checked now that the carrier bearing has been replaced.