The shop confirmed it's the cam sensor and he got the part# from the sensor itself. He said it's not a seal issue and that some other part of the sensor is leaking through the plastic and will need to replace the sensor. Would you be able to point me to the aftermarket sensor you're referring to? I found one online at AutoZone for $199.99. It's a 1994, not sure if it's the same on a 1991. Here's the link:
http://www.autozone.com/autozone/par...5800_652113_0_