Transmission problems?

[Toxicity]

Alright, so we got my 87 Sierra fired up today and the engine worked great. However, when you put it in gear and give it gas, all it does is creep forward. And not just in Drive, in Reverse and Neutral too. The engine revs up, but all the truck does is creep forward realllly slow. Also, the guy we bought it from said it had just quit working on him one day with no noise or anything, so we don't think it's the rear end gone out. Anyone have any ideas or possible solutions? And if anyone cares it's a 700R4 in a 3/4 ton.