I was able to top off the tranny fluid and drive the truck about 5 miles with no leaks and no slipping (yeah!!). Anyone I had spoken to about this reinforced my opinion that the truck's self healing isn't likely to last forever, so I'm pulling the transmission and changing the front seal - with the understanding that with a quarter of a million miles, its not going to last forever, no matter what I do.
I drained the fluid from the torque converter and dropped the oil pan. The oil still had a bit of red color to it and was kind of thin, but it didn't smell burnt or funky. There was a bunch of sludge around the magnet though.
Here's a picture.
So far, I'm replacing the front seal, filter, pan gasket and fluid. I'm probably going to replace the cooler lines since they're a bit rusty. Any suggestions as to what to look for or what else to replace while I'm in here?