I'm looking at buying a 2002 F-250 7.3. my question is how hard is it to regear the rear end so it turns both tires?

 

you mean a locker?

its not to difficult if your mechanically inclined.

 

Changing from open rear end to limited slip? I wouldn't even touch it unless you are REALLY mechanically inclined.

Ex: able to setup and read a dial indicator, run a gear pattern, properly set pinion bearing preload, etc etc

 

It's not for your average backyard mechanic.