Won't shift out of park

GMC vehicles of that vintage had a safety switch
attached to the metal brackets under the dashboard that had a plastic plunger actuated by your pushing on the brake pedal to allow the shift mechanism to move out of park.
This prevented starting the car in a drive gear.
You had to push the brake pedal first before it would shift into gear.
On mine, I found the black vacuum hose very sensitive, and have had to remove the panel under the steering wheel so I could access that switch whenever mine did the same as yours.
God bless your efforts.

Hi JoAnn,sounds like you have a bad neutral safety switch.

• Neutral safety switch sends a signal to the on-board computer to allow the engine to start and to engage the transmission
• When not replaced, vehicle may experience no start and/or transmission will not shift out of park.Here's alink on how to replace it.