Please help! Torque converter.

That is a good amount of miles and how many did you put on the truck?
Have you towed/hauled with it since you have owned it, if so what kind of loads?

The torque is constantly engaging and disengaging and if the previous owner ran it hot or hard (hauling heavy loads OD) there could be damage to the TC.

What does the fluid smell like?

I would be dropping the pan to see what the screen and magnets look like. If it is in fact TC shudder you may be able to get some more miles out of it by servicing the transmission and replacing the fluid with fresh ATF and a friction modifier like Lubegard Shudder Fix.

Be aware tho as the post above me states it could very well be a electrical gremlin too. Hard to diagnose problems over the web forums.
If you have access to a buddy or a local shop that you are in with and they have a scanner that is able to bring up the values for the transmission especially the torque converter lockup I would go for a test drive and watch those values as it is shuddering.

I would start putting back for a rebuild and trying to find a good local reputable transmission shop to do the overhaul and have the aftermarket cooler added and radiator taken out of the loop or replaced with a new rad.