When you lose power on the highway, is there no power at all, i.e., it drops to idle and no matter how much gas you give it it doesn't respond? If this is the case it is likely a speed sensor.

Sensor issues usually take a long time to register a code, since they only fail every X amount of cycles. I had a speed sensor do this, in the beginning it was about every 600 miles. and of course wouldn't produce the problem the 3 times I took it to the dealer. It took about 7 months before it finally threw a code.

I think you have more than one sensor issue, the same sensor shouldn't cause a problem at idle and at highway speed. The TPS really shouldn't be the cause of both problems.

Since you don't plan on ditching the truck, and don't feel safe using it for for what you need, or want to risk driving it for months until the problem manifests itself with a code, you really don't have much of a choice but to start replacing sensors. I would start with the cheapest ones that could be the issue, that is after a tune-up(new cap/rotor, plugs, plug wires, valve adjustment, cleaning the PCV and EGR valves etc.). You may also want to pull the injectors and have them rebuilt/cleaned/tested, you could have a problem w/fuel delivery under a heavy load.