PROBLEM SOLVED!!!!
While comparing the two boxes in Control Panel (there were subtle differences due to one box being Win7 and the other one Win10), I noticed that the troublesome box had an added tab for SetPoint, while the other box did not. I had installed the latest version of SetPoint a couple weeks ago (aha!) to get the Google Chrome extension for smoother online scrolling. I realized that most of my time since then was on the road, so I hadn't been CADing during that time, and didn't notice the panning problem until I got back. Long story short, I uninstalled SetPoint completely, and my problem disappeared with it. I may try to get it working with a SetPoint install later, but for now, I'm just glad to have my PAN button back again!
Thanks, you guys, for the help!