Thanks for the tips fellas.
Unfortunately, this crack is indicative of much more damage inside. Lesson learned: CHANGE YOUR LU OIL BEFORE WINTER. Even if you think it's good and don't think there should be any water in it. I must have had a seal go bad on one of my last few outings of the year last year, leading to water in the lower over the winter.
The good news is that I WAS able to find a used lower unit on Ebay for $175 plus shipping. The motor is a 1973 Merc Model 200 20hp. I was shocked to find one, and as far as I can tell, this is the only one available on the internet. Should be back in action by the end of the week!
For the cash strapped among us, a blown lower unit can mean a season off the water, and I wasn't ready to give in to that yet.