I disagree on last post. 4spd is shorter, and requires a two-pc drive shaft too. Is yours the ball end for tork tube type or the flanged type with park brake on it, or has a yoke on it? If has yoke or flange, will need a slip joint somewhere, so needs second joint for that. Even the tork tube shafts had two pcs. I think if you have tork tube with 3spd, you would need whole rear from a 4spd set-up. I would go with 4spd, change the rear to newer open line, get two pc shaft and make x-over to hold mid-joint. If you look in an assembly manual for the driveshaft from a short COE (5100model), you will see a sort-of one pc shaft with slip joint on trans end that is just like joint usually put in middle. That would be another possible method of attaching.
