First, unless you gave them specific guarantees or warranties (which may have to be in writing depending on OH laws) the sale is considered "as is", and as shortbus stated you don't owe them anything.
As the their claim:
1- was there a noticeable oil leak under the vehicle that they ignored? An oil leak can be a symptom of many things. A rear main seal leak is not necessarily caused by the crank moving or "walking".
2- If the crank was moving as badly as they claim they should have been able to hear the distinct noise from the bearing(s) when they test ran/drove the vehicle. If they couldn't hear it, it wasn't that bad (if it was bad at all), and they had opportunity to have it inspected further if they had any doubts.
3- Unless you have had experience with engines/crankshafts that are deteriorating you wouldn't know what (if) there was anything wrong. If you represented yourself as an expert mechanic who was claiming that the engine had been thoroughly checked and had no unusual wear or defects your buyer might have an ice cube's chance in Hell of getting a judgement in court...and that would depend on OH's laws regarding disclosure and reliance on your professional/knowledgeable status. I am assuming you are a private party and not a dealer.
4- You didn't mention how long they have had the vehicle. A 4X4 truck would be considered as a vehicle that would be expected to see heavy or hard use. They could claim all they did was drive it home on smooth streets, but if any amount of time has elapsed it would be reasonable to assume that they have taken the vehicle out off road to "test" it. Logic follows that they could easily have caused damage to the engine by overrevving or even running with low oil. That puts the ball even further back in their court.
In Ohio State football language your buyer is on his own 3 yard line with a sprained thumb on his throwing arm and no decent running backs.
Unless you feel some moral obligation....like these were old hunting/drinking buddies....you really don't owe them anything. As to the cost: yeah, it would involve removal of the engine and a rebuild (replacing only the crank would be futile) to put the truck back into useable condition. $2K easy.
