best guess is a broken flex plate.
Big miles = look carefully before you spend any money to fix the truck
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Thanks. Do you have a guess on what the repair will cost and how long it will continue to run if I don't fix it?
Cost is hard for me to calculate. You are looking at removal and replacement of the transmission, plus parts. There is also the possibility that this is an engine bearing issue. Your best bet is to take it to the mechanic, and let him have a listen. As far as how long it will go, can't say. The longer you let it go, the more chance of the problem doing some extra and expensive damage.
Honestly, with this kind of miles, be super careful about repairs, because on a high miler the costs can spiral out of control quickly.
Junked the vehicle. $1800 in parts and labor to fix it.