I have a 1997 Dodge ram 1500 with a 5.2 L it is a 2 Wheel Drive, i’ve been having this issue where if I have the bed on the truck the truck will not start. But if I take the bed off the truck will start just fine. I replaced the fuel pump and it checked timing and spark on the truck.

 

So it only cranks? Do you get fuel to the rail? Have you measured fuel pressure?
Only that would make sense is that something gets pinched when the bed is on the frame, either fuel line or fuel pump wires are broken

 

Likely the harness is getting pinched when you set the bed down. Check the routing. Look for the flat spot.

 

I like the fuel pressure check plan. That would indicate just where the problem is.