Check the fluid levels in the axles first, look at the drive shafts, check for play in the u joints and the pinion bearings, lift the wheels individually and check for "play" in the wheel bearings, etc. Exhaust all possibilities before opening up and pulling things apart.
If all seems OK, then take the differential covers off and take a look. It would be odd that the knock is speed specific.
Does the truck shake/vibrate too on acceleration?