I have an 86 Extra Cab w/carb which shudders badly just before upshifting to the next gear. I have changed plugs, plug wires, distributer cap and rotor with no change. This is a bad shudder, the whole truck jerks. Could this be a vacuum leak?

 

Welcome to YotaTech.. are you sure its a driveability shudder with the engine. Could be a driveline vibration that becomes noticeable at 2500 to 3500 rpm, typical shift point rpm. Check u-joints, output shaft and pinion flange for excessive play.
Definately not a vacuum leak.