Here's some new stuff about my coolant system problem:
I checked my overflow tank this morning (when the engine was cool) and it was entirely filled, all the way up to the lid...way past the "cold fill" line. I also checked the coolant level in the radiator reservoir and it was very low. Keep in mind that my coolant levels were all fine the day before (the overflow tank level and radiator level were filled correctly before I drove). This is telling me that coolant is expelling into the overflow tank but not being sucked back into the radiator. I'm starting to wonder if the radiator cap is working correctly; maybe it's not adjusted to the right psi...maybe it's not the right cap for my truck in the first place. Can anyone tell me how this overflow tank problem might be related to my coolant system problem?