Update: Working!
Hey all PTers out there.
Here's the scoop:
2.16 GHz MBP
2 GB RAM
MBox 2
OS X 10.5 (Leopard!)
PT 7.3.1cs5
After considering reinstalling Leopard with a clean install in order to truly see whether PT is non-functional with Leopard, I decided to take a different route.
After an extensive open-close-open-close exercise, dragging and dropping each of the Plug-ins from a relocated folder into the active PT Plug-in folder (Library--App Support--Digidesign, etc) to test which will of the plug-ins causes problems, I have discovered that ReWire is the culprit! No other plug-ins have caused PT to crash while opening, and all of my plugins have operated very well in Leopard.
Solution: for now, if you'd like to use PT in Leopard, simply remove the ReWire plugin (and keep it in a safe spot just in case).
Please, others that try this respond with your results! Cheers. 😀