I wouldn't give up on the search yet. Go ahead and drive the one you posted about. Drive it as long as they will let you. Drive it how you normally drive. Open it up a few times see how it reacts, etc. It could still be a good deal, I would just put it through the paces and really study it well.
The V6s arent horrible. I've had 2 and the last one I pulled and sold to go into another one. Pretty peppy for a 6 as long as the trans is right. I've pulled a little with both and its not bad, I pulled a ext cab f150 out of the mud with a trailer on it without much trouble at all. They do have timing chain tensioner rattles, (they all will do it) and thermostat housings are a joke, (replaced on both trucks) Fortunately, a member here has an aluminum housing they are making and selling now, which I plan to try if I keep the 4.0 much longer. Other than that, Im fairly happy with the 4.0 for what it is....
That being said, If possible, hold out for the V8. More torque and Hp, with pretty much same mileage. (providing you dont favor the go pedal more than you would normally in a 4.0 truck.) Dont get me wrong, The 4.0 will cruise 80 all day long with no issues, but for towing, passing, hauling lots of people, I would say V8 all day long. You get 5-7 people in this thing the V6 will whine like a baby trying to do any better than a granny drive. To be expected towing somethin large/heavy, but intended occupant capacity Im not too impressed. My current truck has 253500 miles with a 3 year old crate engine, your results may vary.
Whatever you do, Don't give up quick, and dont buy after a first drive.
Last thing--- Im a Chevy guy, so this should tell you something about these trucks and my "review". Although, I plan someday to throw a 6.0 or 6.2 Chevy into the Mounty or buy another one to do it to.....