Intermittent tail light problems?

Sounds like there's a bad ground, either in the socket or in the wire. When the problem occurs, remove the bulbs from the left side. If the right ones go out, there's your proof of a bad ground. You will also be able to measure a voltage on every terminal on the right side bulbs. If you have the round brass-base bulbs, there should not be any voltage on that base. If there is, temporarily touch a jumper wire to that base and to something metal on the truck and you'll see the lights start to work. If you have the newer flat plastic-base bulbs, figuring out which one is the ground is a little harder.