1981 Jeep Wagoneer out of gear

Hi: My transmission will go into gear and shift properly when engine is cold--when it warms up it will slip out of gear and engine will rev but no forward or reverse motion. turn off engine--wait for it to cool--will go into gear and and drive for a short while then slip out of gear and start problem all over again. Is it terminal or is there a quick fix. Thank you, Bruce