If you are slipping in 3 or 4, it could either be the 3/4 clutch pack burned up, or Forward Sprag failure. If you really wanted to, you could pull the trans, drop valvebody, pull pump, and everything for 3/4 is in the front.

But that doesn't explain why it went bad in the first place. If I was near some pappers there was either something about valvebody gasket failure that causes some problem, or leaky shift solonoid. Don't quote me on those though.