My 1994 Ford Thunderbird Won't Shift Gears?

So my T-Bird has been having some transmission problems since I got it. It was mostly minor, shuddering between gears and such, but the only problem worth mentioning was that the fourth gear seemed to slip. I could use the overdrive button to make it work for a while, but now it doesn't seem to work at all. I start the car, and I can move the gearshifter out of Park, but it doesn't engage in any other gear. I thought maybe it was the interlock system, but the trick with starting the car in Neutral didn't help at all. I've already checked the fuses, battery line and transmission fluid, so I don't know where else the problem could be. Any help?

Well if transmission fluid full and it goes through all the gears and won't move then internal problem. The slipping you had was a internal problem and now what ever went inside went. Option now is it needs rebuilt. Find local shop at ATRA.COM for more help