Mud Chigger Problem
This depends a lot on if it is a pressure or vacuum on the tank. The exhaust heating the tank is feasible but if the same problem is happening after only a few seconds of running then that is likely not the problem. I think there is a vent problem for the tank. The tank should always be at the same pressure as the air around truck. If there is a vacuum it is because there is not enough venting to the tank. Most of the venting occurs through the charcoal canister. If the tank is not venting, as the engine runs it will draw a vacuum on the tank. If the vacuum is high enough it is probably pulling vacuum through the canister. The canister is connected to the tank and the fuel system to catch vapors from the system so they don't get released to the environment. Hence, if the canister is clogged, the vacuum will pull through the canister, through the fuel system, back to the fuel pick-up line from the tank, and dump it through the overflow. An easy check is disconnect all of the lines on the charcoal canister, put the cap back, and see if the problem persists. If it does not, replace the canister.