What pin on the OBD2 port could I run the tach lead off of

There is no tach lead anymore. Digital signals transmit that information between the various computers, and the instrument cluster is a computer.

Since you even have a distributor, I'm assuming you have the 3.0L V-6 engine. They use an optical sensor that develops many pulses per crankshaft revolution. The crankshaft position sensor develops 9 or 12 pulses per crankshaft revolution, depending on the year, but they aren't evenly spaced. They show up in three distinct bursts.

I can only think of two possible ways to add a tach. One is to trigger off a single spark plug wire if your tach has a setting for a single cylinder. The other is to head to a salvage yard and get a cluster with the factory tach in it. About 95 percent of Caravans came with the tach so you won't have any trouble finding one. I never tried to do that swap so I don't know for sure if the connectors are the same. Engine speed information is used by the Body Computer which talks to the Engine Computer and Transmission Computer, so you know that information is always there on the two-wire "data buss". The instrument cluster will know what to do with it.