Update time, ladies and gents.
1. I found my issue. There may or may not have been an installation error regarding the rag joint. Installing it properly may or may not resolve steering issues. I'm not willing to admit to anything.
2. I believe the 98 XJ shaft splines will indeed match up.
3. Even though the rag joint is now installed properly, likely fixing my issue, I'm still going to upgrade to the XJ shaft end. I checked my upper joint at the column and it appears to be in decent shape. When I receive the end, I plan to do a short test drive with the rag joint installed, swap to the XJ joint, then repeat the same test drive and note any differences. I'll post back with a conclusion in a week or so.
Here we can see the splines on the rag joint....
...and here we can see the XJ U-Joint.
Here's an overlay of the 2 with 50% opacity. I didn't rotate the image to match the "flat" spot on purpose. I wanted the splines to line up properly. You can see they match up nicely.
Lastly, here's a zoomed out picture. Notice anything (apart from the oil sludge buildup)?