There are several reasons. Two common reasons would be warped brake rotors or a bad ball joint which should be fixed immediatly.
Check for vacuum leak
I'm having this problem currently with mine. Someone told me it might be needing new cvc joints.
apply your brakes gently while accelerating at the same time
Most common cause of that is the rotors are out of round are warped. REPLACE ROTORS.
i believe it has rear drum brakes so it could possibly be a warped drum or back shock or strut those would be a good place to start
You did not have to apply the brake on a 91 model to shift it. There's nothing wrong.
Time for new brakes or adjustment.
it broke
apply your brakes gently as you accelerate
apply your brakes gently as you accelerate
What is stab braking?Apply your brakes all the way.Release brakes when wheels lock up.As soon as the wheels start rolling, apply the brakes fully again. (It can take up to one secondfor the wheels to start rolling after you release the brakes. If you re-apply the brakes before thewheels start rolling, the vehicle won't straighten out.)This is directly from the NYS CDL Manual.
The parking brake on these vehicles are basically self-adjusting. Fully apply and release the hand parking brake lever 4-6 times to self-adjust.