So, I have owned a few of the old Jeep Cherokee XJ series SUVs. I have seen parts of these jeeps that probably only 10% of owners have and at this point i could become a certified XJ mechanic. From the sounds of it, you have dug so deep into your Sport Trac that you could easily be within that 10%. Heck, probably within the top 1% if you have done your own transmission work!
I have made working on these XJ jeeps into a small side business, buy them cheap (think $500), fix them to the point of reliable/serviceable and then sell them for a nice bit of cash. Your experience on your ST is worth more than the parts you have bought and maybe even the truck itself, perhaps you could make this stinky lemon of a pile of heap Sport Trac into a weekend side gig. It sounds like you have the time, space, and tools to do the work! Now you have the know how on finding a clunker and turning it into a solid sell. Worst case is you find a truck as bad as the one you got now that needs a ton of work. Best case... lets just say I bought a jeep once for $2000 and sold it a month later for $4000. Person I bought it from was very grateful, didn't have the space. Person I sold it to couldn't be happier to find such a good condition 4x4 with low miles. Sometimes everyone wins.