indianajones

My 4 speed a/t has suddenly become a 2 speed, what should I check first? It'll go into first gear when I put it in D, but won't shift out. I can get second gear by dropping down to L, but it won't shift any farther no matter what I do, does anybody know what's going on? I read something about a vacuum modulator valve or something, could that be it? I can't keep driving around in 2nd, any help would be greatly appreciated---thx