Why does this truck only runs in reverse?

[eee]

My parents have a 1977 chevy silverado that has been sitting in our yard for the last decade. A few years ago, my dad started it (It still ran, to his surprise) to try to give it away. He drove it forward about 10 ft. and the transmission died. Now it only runs in reverse, or at least it did a few years ago. Does anyone have any ideas to what the main problem might be so it can be fixed?