Found it!
I checked & re-checked suspension, exhaust etc. Took out the hitch and pin. No luck. Ordered new shocks in case I had a bad one (still running the original ones). Have not arrived yet so the search continued.
Found a loose battery operated light I had mounted in the bed for illumination with the camper shell. Removed light and rattle went away.
My son (12 year old) was the one who took off the clamps on the shell at the front of the bed with the air ratchet while I took off the rear with the regular ratchet. He did not know to check the mounted light and I never thought to have him check it. Should have used the KISS method on eliminating possible sources. Oh, well, rattle is gone. Thanks for all the suggestions from fellow members.