electrical failure?

Hey gang, After 271,000 miles on my '86 4WD pickup I finally have a problem I didn't cause myself (I think). Two days ago the truck started right up and ran fine...the next morning, the battery was totally dead. Nothing was left on to drain it so there must have been a short somewhere. Here's the kicker...when I tried to jump it, both batter terminals smoke. When I tried to hook up a new battery it just about melted the terminals and still wouldn't turn over. Any guesses as to what's wrong here? Appreciate the help as always. Oh...enjoyed the posts about the Toyota reputation...I had to change a tail light lamp once also...