I am not certain of the pedigree of your PTO driveline as many things can happen to old trucks over the years. On the old original systems I am familiar with, the output shaft u-joint yoke is attached to the output shaft but a 5/16 or 3/8 inch diameter set screw. Loosen the set screw and carefully slide/pry the yoke away from the PTO. The driveline should have a slip joint in it (typically near the center bearing) allowing for some movement aiding in disassembly.
