You might be able to pull 15k but 13.5 is a pretty serious offer. All the bushings and belts, fluids will have to most likely be replaced anyways. If you need a car, keep it an drive the crap out of it.
Why would you be skeptical of the mileage? There are tons of low mileage examples of these, plus it's a 2002 that's over 8k a year, that's pretty reasonable.
Sounds like it's for the mall crawler crowd with too much money to spend. Too many Jeeps rolling around with $20K+ in modifications w/o one scratch on the paint as it is.
It's the fact he has a spooled front diff...simple as that. Plus it's pointless to trouble shoot since it only happens on pavement which you can't drive a locked front in 4wd on pavement anyways.
Are your tires the exact diameter? If one is more bald than the other or even different air pressure I would suspect it would reek havoc with a welded differential.
Maybe look into what a spooled differential is, then try and figure out why you are having problems. You have cheaply locked front axle, of course you are going to have problems on the street.
Why would I be mad? People are dumb with money. For example the people paying 30k+ for 4th-5th gens with 60k+miles on them...why not just buy a new one for a couple grand more? I'll never understand. Congrats on selling it.