1995 Geo Prizm wont change out of 2nd gear

I have a 95 geo prizm. The issue I was having was the tranny so I put in a motor and tranny from a 96 and it was fine at first. I drove home from work (29 miles) and I parked it for about a week. When I went to drive it the tranny wont shift out of gear. When I accelerate the car passed 20 mph it revs high. When I let of the throttle it does nothing until I rev the rpms back up. The car runs great otherwise. Any and all ideas wuold be greatly appreaciated.