RedExpy, first thing that I will recommend to you is to get the throttlebody out to a spot that you can work on it. So, remove the throttlebody from the truck. I know this is more work, but from the sounds of things, this is going to be the easier way.
From there, you have a few options, it is all in a matter of how you want to proceed. I would personally recommend getting something like Kroil or PB Blaster. WD40 is good, but this stuff is better. Also, spray the stuck bolt and then let it sit for 20 minutes or so to let the oil soak in. From there, see if you can remove the screw. If not, you are more than likely going to end up cutting the head of the screw off. This is where having access to something like a dremel tool will be best. Once you get the screw head off, you can slide the TPS out of the way and use a pair of vice grips to go after what is left of the screw. Don't forget to keep applying a little bit of the Kroil or PB Blaster to help let it get in there and dissolve any sort of rust. Keep twisting and fighting to get it free. You can also try using a hammer to strike the end of the screw while holding the throttlebody solid. This jarring action will help break the rust free and can possibly help the screw come out. Using a punch of some sort can help so you don't strike the TPS.
If you reach a point where the screw just isn't going to come out, then you are looking at having to drill out the screw. Due to the size of the screw, there really is only 1 way to fix this. You are going to need some sort of device to hold the throttlebody, a claw hammer, centering punch, various size drill bits, drill, and a tap that is the same thread as the screw that you are drilling out. Use the dremel tool to cut off the screw flush with the throttlebody. Using the hammer and centering punch, put a small dimple in the center of the screw. Now, using a 1/16" drill bit, drill a hole down the center of the bolt. Be as precise as you can. So, holding the throttlebody still will be important and having a steady hand with the drill will be too. Drill down to make a hole through the screw. Step up to a larger drill bit, but one that is still smaller than the threads of the screw. Enlarge the whole. Repeat until you have drilled a hole that is the diameter of the center of the screw (so all that is really left is the threads of the screw in the hole). Using the tap, run it down the hole to pull the threads out. Don't forget to spin the tap 1 turn or so, stop, back it up a little bit (1/4 turn or so) then make another 1 turn. Repeat until you have removed the screw or reached the bottom of the hole.
