92 GP - trans locks up in reverse AND park

Hi, On my 92 Grand Prix 3.1L,auto trans w/overdrive (3T40?).When put in reverse, car will move a few inches then "lock up". It seems to be in reverse and park at the same time. Giving gas, I can feel the car trying to move backward, but is held in place. Once it is in this mode it remains locked even in neutral. Placing it in any forward gear and then moving up a few inches releases it. Then after going into reverse or attempting to roll backward in neutral, the trans will lock up in park and stay that way till placed in forward gear. Fluid changed a few weeks ago-correct fluid and level. THANKS for any suggestions.