I would keep it and make the normal repairs and maintenance one would on a 100k 4th gen Explorer/Mountaineer. The main reason is the drive train in your model being a V8 makes it very reliable if maintained. Especially since you are the original owner and know its history. The biggest thing to do now apart from the ball joints is fluid maintenance. The most important maintenance items to do right away would be a coolant flush, transmission pan drop/replace filter/refill (do not do a transmission flush) and transfer case fluid drain/refill. Then follow up over the next 10k miles with front/rear differential oil drain/refill, brake fluid replacement and power steering system flush. Don't rely on Ford's recommended service intervals for fluids as they are ridiculously long for the items I referenced. Then do a transmission pan drop and transfer case drain/refill every 30k-40k miles going forward. This, and 5k oil changes, should let you get many, many more miles out of the vehicle's drive train. Also, change the original 20 psi radiator cap to one rated for 16 psi. Many here have done this with no issues (including myself) and it will likely prevent some cooling system failures going forward. This is due to 20% less operating pressure in the cooling system unnecessarily stressing these components. The 16 psi cap can be bought here for around $15:
16 psi radiator cap . You can change it yourself when the engine is cold.
There are other items that can need replaced like a wheel bearing, or two, over the next 100k miles. Radiators is another but the radiator cap replacement might prevent this from occurring. The good thing is your model of Mountaineer with the V8 is very reliable overall. Most repairs you will see will be less than a monthly car payment on a well optioned new full size SUV (assuming your Mountaineer is paid off). Plus, the 2006-2010 Explorers and Mountaineers are the last ones to be share the same drive train and frame as the F150 pickup truck and is more robust than the newer models.
I opted to buy the 2010 V8 Mountaineer I have now over a 5th gen Explorer because they had engine reliability issues and engineering design I did not care for. I instead bought a 100k mile V8 Mountaineer loaded with all options in very good condition overall. I knew it needed a new radiator, tires and the maintenance done that I listed which cost me around $2k on top of a purchase price of $6.5k. It drives like a dream and I am very glad I bought it. So my advice it to keep it, maintain it and do the reasonable repairs likely needed to keep it going. You should get another 100k miles from it easily if you do the maintenance I suggested and the repairs that are typically needed over the next 100k miles.
Lastly, the only reason I would say to sell it is if it has severe rust occurring in the body panels or undercarriage. I notice you are located in Illinois where this could be an issue. I grew up in Indiana and the salt they apply to the roads in winter could do a lot of damage to cars.
