The OP made multiple poor decisions:
*Private party sale with no dealer backing.
*Mostly or all financed.
*High mileage vehicle.
*No warranty.
*Multiple questionable modifications.
*Vehicle model with dubious reliability reports.
*OP likely not mechanically inclined or no mechanic inspection.
OP, is there any chance you recently visited a “Jiffy Lube” and maybe the oil change tech didn’t torque your drain plug, which then fell out, leaked oil everywhere, and oil starved a perfectly good engine until it seized?
Otherwise, I got nothing else. Depending on your assets/income decide whether to default on the loan or just park the truck in front of your home and hope an alien spacecraft craters it like the pickup in Men In Black, then call All State.
Sometimes, the biggest contribution a person makes is being a lesson for others. This is a rough one. I hope it was an oil jockey at Jiffy Lube that messed up and you have some recourse, but I suspect the seller sold you a known time bomb.
