Well I think I've found the problem. Take a look at this picture:
When I manually move the driver's wheel the rack itself moves up and down (the direction of the red arrow). The whole assembly is nice and tight to the frame, the linkage has virtually no slop, and the extra anchoring I put on the column is still holding the column in place tighly. So it looks like the new/rebuilt rack and pinion assembly I bought a number of years ago is bad. Looks like I'll be ordering up a Flaming River rack from Summit Racing.
Thanks for all of the help everyone!!!