Is it worth fixing?

We bought our son an older 90 ford f150 v8 4x4 with rebuilt transmission for this past Christmas, he loves the truck but it is not in that great of shape "rusting a little here and there" "worn paint" and such. It is strong and heavy 4x4 works great and engine/transmission we are told are very strong. The problem is that he got it stuck in some deep mud trying to push someone else out and during the process of them trying to get him out they ruined the whole rear end "axles rods seals" which we found out will cost around $1500 to fix. Question? Is she worth fixing? Or should we just sell her as is and try again with another vehicle? She is old well worn but has some great features like lifted 3in with nice tires rear sliding window good 4x4 and is a v8 new stereo w/mp3 usb and strong motor, we paid $1900 for her which I thought was a steal until we had to pay another $850 to get her to pass emissions plus $200 for stereo and a couple trips to the mechanic for minor things. In total we have probly spent around $4000 and now $1500 for the rear. My son is going off to college next year and not planning on taking it with him as he won't need it. Advise? He does love that truck though, keep that in mind.