Automatic transmission will only shift at high.

Check the fluid level.
When was the last time the fluid was changed?

Check the vacuum line to the vacuum modulator valve. To check it, look on the pass side of the trans towards the rear for a small vacuum actuator. It should have a vacuum line which runs from it to the intake manifold. It and it has one bolt holding it on to the trans.
Make sure this line is connected and it have vacuum on it when the engine is running.