Sure sounds like a ground problem but here's a couple of things you can check:
You're gonna need either a 12 volt test light (these work great for troubleshooting these kinds of problems) or a voltmeter.
First, pull your switch out to the first notch. Since your license plate light works, and I believe the license plate wire is tied in with the tail light circuit, I think we can assume that your fuse, switch, and wiring to the back of the truck is good. So, take one of the bulbs out (now's a good time to make sure the bulb is good!!!) and look into the socket, you should see 2 points that contact the bulb. Connect one side of your test light, or meter, to a GOOD ground and probe the 2 contacts. One of them should be hot. If neither of them give you a reading, then trace the wiring back towards the light switch in the dash but I'm betting you will get a reading.
If one of them is hot, move the probe from ground and clip it to the metal light socket - you should also get a reading here. If you do, look carefully at the socket at make sure the contact springs are pushing the contacts towards the bulb, and that the little wafer hasn't twisted in the socket so that the contacts aren't making a connection to the bulb.
If you didn't get a reading when the ground probe was moved to the housing, you've got a grounding problem. Make sure the housing is grounded good to the bed, and make sure the bed is grounded to the chassis.
Sorry for the novel, hope this makes sense, and good luck, keep us posted.
