I was able to get the difference between the top and the bottom of the image down to about half of what it used to be.
I spent a lot of time fussing around with shims and putting them in all different places with varying amounts of success. Finally, I took off the thumbscrew that tightens the X-axis adjustment of the lens board and found that there were two thin washers sandwiched between the frame and the enlarger chassis.
"Aha!" I thought. It looks like somebody designed the enlarger to be adjustable by changing the number of washers in there.
I just happened to have a bunch of that exact kind of washer my toolbox, out in the garage. I got a few of them and was able to improve it. I only have to turn the knob by half of what I had to before in order to focus the top or the bottom.
I'm getting tired and a bit frustrated. I'll look at it tomorrow or the next day when I can do it with a fresh mind.
Cross my fingers... This might be fixed!