I figured it out - the pin is directional and seats once fully inserted. I was trying to knock it out the wrong way - go figure!
After further consideration I don't think I'll be able to get the shaft all the way out without removing the intake manifold... I think I'm just going to bring the truck to a pump shop and have them swap it out for a rebuild. I'm going to start another thread now to ask for suggestions on what else to have them look at / replace while they're in there with the manifold and pump off.
1997 Chevy P30 6.5L N.A. "Y" VIN w/ 4L80E & 4.10 Rear - Grumman/Olson Step-Van Body