I took a few pics to go along with my description.
Here is the underside. I can see water leaking down the pipe, which collects on the U fitting and drips. This seems to only occur when the tub has at least some amount of water in it... like about half way up to the overflow. It also leaks sometimes when someone is taking a shower, but does not always happen.
Here is the drain.
I am thinking about trying to remove the drain and re-sealing it. Otherwise I'm afraid I'm going to have to cut out the section of wall behind the plumbing of the tub to see what is going on. Tips anyone?