All one needs to do to bypass it is turn the key to the unlock position (not RUN, but UNLOCK), shift to neutral, then start the vehicle and drop it into gear.
There can be a few different causes depending on the year and model of truck. A common cause is the SCDS shorting out, and blowing the fuse for the brake lights. On some years, the brake-shift interlock is fed from the same feed that powers the SCDS and rear brake lights. The third brake light will usually still work on those vehicles.
There are other causes, including mechanical failure of the interlock, a bad brake pedal switch, or other blown fuses and wiring issues.
Basically, the interlock looks for two power sources to release: Brakes, and switched ignition power. Leave them both off or both on, and you can bypass the interlock. If you have input from only one, (ignition or brakes) the pin locks the shifter.