From what I remember, the XJ trick will not work on pre-77 GM trucks. What you want is a splined connection at the steering box, and a through-bolted, flat-sided shaft sticking out of the firewall of your GM truck.
The Jeep shaft should come from a XJ Cherokee, not Grand Cherokee, year model 88-96.
Pro Tip #235 - There is a white nylon friction bushing in the Jeep shaft assembly. Some folks advocate heating up shaft w/ torch, melting this bushing out, to make the install easier. You actually want this bushing in your final Squarebody install. If you goto my Reference Library you will see me stretching out the shaft using a ratchet strap (do not hammer it w/ steel hammer, you will damage u-joint). After stretching it out into two pieces, use some sand paper or grinder to GENTLY sand down the nylon bushing a little to make installation easier.