If you have tires with deep tread and drive on gravel it could be rocks being thrown up. After putting new ones on one of our trucks it does that. It is a dually and one time it had a massive rock between the tires and it flew up and sounded like a shotgun blast in the cab.

 

Check the leaf spring bracket bushings...mine were shot and even had some gap and a lot of play in them. Mine was making the same noise.

 

I had a similar issue several years ago. Ended up o.e of the rear leafs in the spring pack had broken in half. Found out as i was driving down the highway when suddenly half a leaf spri.g was bouncing down the road behind me, lucky the were no cars behind me at the time.

 

Do you have overload springs on the rear?
If so, going over a sharp bump the axle will jump higher than usual & the overload springs hit the stops with a 'clank'.

It honestly took me a couple of years to figure this out on my truck , because it was intermittent & everything under the truck was tight & solid.

 

When I bought my 88, I also had a lot of noise from the bed area, and I pretty much fixed by:
1. Tightening the spare wheel carrier (I know you don't have one).
2. Replacing the sway bar bushings - they were all shrunken and allowed the bar to flop around.
3. Replaced the shocks - besides being worn out, their bushings were also shot and the shocks would bang agains the bushings quite loudly.
4. Wrapped the tailgate latch pins with electrical tape so it couldn't rattle around.

I also went around and verified that every nut and bolt was tightened to proper specs.

The last thing I am in the process of is replacing the U-Joints which are pretty bad, but I don't think they make much noise.