Trany won't engage

My 2003 Explorer which I'm trying to sell is giving me a hard time. Guess it knows I'm trying to get rid of it! I drove it home with no problem and now it will no engage in any gear. I had it rebuilt about three yrs ago and everything was great for quite a while until it started flairing from 2nd to 3rd. I tried the aftermarket servo with the o rings and it really didn't help but we learned how to drive it like that. Just for the heck of it I checked the fluid level cuz I thought it might be low but to my surprise quite a bit of fluid ran out so I let it stabilize and it ran better until I parked it last. I checked it again and more fluid ran out.Now I'm wondering if something isn't doing it's job in there and not sending fluid to the converter. Any ideas?