I'd ask if you were kidding on the $400, but I guess I don't actually want to know -ouch- .... On my 1948 I changed the old rock crusher tranny to a T5 (out of an S10, there was no world class mustangs around), and had a driveshaft shop MAKE me a new one - from scratch and it was less than $400 - totally custom, old knuckle style on the back, new spline slip on the front, non-standard length everything... all I had to do was supply the length measured to the nearest 1/8th inch. I think it was even less than $500 after taxes and parts (I had to pay for knuckle,spline section etc).
Hell if you were willing to wait for a nice warm day, give me the length exactly (for verification), pay cost/shipping/$20 in gas (the yard is an hour away from me), I would go pull one of those $20 driveshafts out of the 'yard and send it to you. I know there is an '89 in there extend cab with something smaller than a 2.9 (I took its front bumper for my fixing and only glanced at the engine) and 3 or 4 other 2nd gens... maybe even the right length shaft, every single thing in the yard was 6' bed... lots of 3rd gen, few 2nd and 3 or 4 1st gen. I don't really know what engine's were in much of em cause I was looking at bed length as my primary and not one 7' like what I have.
I got another angle for you to take on figuring out the right part... is there nothing left of the shreds with enough of a part number you can guess at the original part number?
Something else for you to do in your spare time if you feel like weeding through 70 different parts online. Oreilly lists 70 hits and the first 2 pages are all coupler (the 3rd page has a few then some acdelco parts and grease guns and total irrelevant junk)
Search oreilly's with NO vehicle filter just the words "driveshaft flex coupler"
then it is just a matter of looking at each one, one by one to eliminate it......ugh