Just as a check to eliminate any clutch or other odd driveline problems...
If you are on flat ground, drop the truck into 1st gear, turn on the on/off switch. Do not push in the clutch pedal or step on the brake and hit the starter button.
The truck should crank and pull itself forward and start...
If the jerking goes on, even when it is just cranking, I would change the u joints...they are all the same and a 369 joint is the part number...that should fit the front, center and rear drive shafts.
It might be quite rough when you crank this way if the ujoints are bad...if they are good, it should spin and start easy with no real jerking.
Let us know what you find.
As far as the brake light switch, it is as said a pressure switch on screwed into the side of the brass block mounted on the frame below the master cylinder where the line from the master goes down to and then splits to the 2 front brake lines and one line to the rear...unfortunately, there does not appear to be a single image in the manuals showing this...
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!