I am fairly certain tha the poor mileage and highway performance (as well as the occassional stalling out of the motor at 2000 RPM) is due to a failed stator in my transmission.

In the process of researching this, I found out the following:

At least from 1984 on, the 904 transmissions in vans all had the wide gear ratio set. If you find an 80s era slant six van with an auto it will be a wide ratio 904. At least in 1986 and 1987 the lockup converter was controlled by the Spark Control Computer, EXCEPT! for California models which had NO lockup!

Interesting stuff. I figured some of the late model truck guys on here would be interested.