A proper driveline angle is most important for a hwy driven truck.
I'm running a cv rear driveshaft and followed the driveline websites to set my pinion angle. It just takes one of those $5-10 angle finders from sears or harbor freight and fairly level driveway or even better the garage floor. Park the truck and take a few readings.
First the slope of the engine / transmission, there are several place you can take the readings. The valve cover or the bottom edge of the 727 transmission. Just looking for a machined edge. Mine transmission was close to 5° tilt to the rear.
Second measure the current pinion angle of the rear end
Now measure the driveline angle. I assume you have one installed.
You want the pinion to point down 1 or 2 degrees from a straight line to the transfer case to account for spring wrap.
When you shift gear the pinion will rotate up. Just look at a lifted truck to see this action. Not that yours is that lifted, but its easy to see what everything is exposed..
All of this info is in the link I posted above.
Edit... If your running a stock dana20 the newer ones have a 26 spline shaft and the 88-93 Jeep cheeroke front drive shaft is a cv, so you can use the yoke off of the Jeep transfer case on the IH Dana 20 if you want. I ran a Jeep driveline and yoke for years on my Scout.