Revisiting this issue, I took my '98 Boxster (39K miles) with Tiptronic for a spin, and everything seemed perfectly fine. I have noticed over the past year that the shifter has been a tad "tighter" but didn't really worry about it. After getting home, I went to move the shifter from D to P, and it was frozen solid in D.
I applied as much force as I felt comfortable with, without damaging anything, to no avail. Thankfully it happened in the garage, so I applied the parking brake, and disconnected the battery since I couldn't turn the key to the off position.
Fast forward to the next day, I checked the operation of the shifter and it slid smoothly into P. I ran the shifter through its paces, and it shifted like butter. It seems the issue is directly related to the transmission being hot. I have not driven it since, for fear of getting stuck somewhere with the transmission stuck in D.
I came across this article, and wonder if this is the issue that I have? I see they only made the specific part out of Bakelite in '97, but it is possible an early '98 might have it? Is it an easily replaceable part?
https://www.transmissiondigest.com/1997-zf-manual-valves/