Hi, My 92 Legacy AWG Turbo Auto 5spd has the button on the shift handle below the big button that you have to push to get it into park or reverse. It sounds EXACTLY what what happens when you push that button in. Of course, if it is the same model, the button or linkage could be broken too!
There are a couple of things to check out there if you find that button is on the shift handle (it is much smaller than the "park"/"reverse") button.
If it is there, see if it works smoothly, and check the linkage. Just some ideas, but sounds EXACTLY like the cause!
The Ole' Factory Rep