It could be the brake booster or the check valve. Start the truck, pump the brakes. If the are still locked after you release the pedal disconnect the vacuum hose to the booster. If the brakes release the check valve is bad. If the brakes are still locked pull the pedal up by hand or foot. If the brakes release the booster is bad.
Basically, no this wouldn't work because your transmission is already setup for a lockup TC. This would just be adding parts you already have and not what you need. If you want your TC to lockup, you need to have a lockup TC. Replacement with the correct TC is going to be your only way forward.