THIS PROBLEM HAS JUST OCCURED WHEN I CHANGED THE BRAKE BOOSTER AND THE MASTER CYLINDER
WONT SHIFT OUT OF PARK I JUST CHANGE THE BRAKE BOOSTER on 1996 GMC Yukon
when you changed the booster, you may have disturbed the adjustment of the brake light switch which allows the shifter to release when you apply the brakes.
Mine did the same thing. I now, without changing the brake booster switch under the steering column and dash, just hit the brake as soon as I try to shift outta park. Its a timing thing. Works everytime.