About two weeks ago, I noticed some liquid dripping off the underside of the truck on the drivers side, just where the rocker panel/floor panel sort of "ends"

I thought it was water because it's been getting cold here, and it's simply condensation. Nope, it's some sort of oily fluid. Clear/yellowish, but I'm fairly certain it's not brake fluid because I don't think there are any lines that run just below the drivers door.
I drive a regular cab 1988 F-250 with the 7.3 IDI. There's some rust there, as well.
If anybody has any helpful insight into finding this mystery leak, or even if someone else is/has experienced this, I would appreciate it hugely.