Did you put rubber shims between the bed and frame. If so you may be losing continuity back to the battery (via the engine ground strap). There are several places where the ground wires leave the harness and are bolted to the frame. The frame is in turn connected back to the negative post on the battery by flexible straps, the mounting fasteners and finally through the black cable that attaches to the battery.
If only the rear end is acting up (and you just removed the bed recently) remove any rubber and paint between the bed sheet metal and the frame. Or run an independent ground strap from the bed to the frame.
The ground wire that is led to the gas fill pipe is not the same animal as the wiring on the negative side of your electrical system. It is only there to dissapate the build up of static electricity during fill-up. It bleeds that charge off into the metal mass of the frame so no sparks occur at the gas nozzle/fill neck interface and blow you up.