[superchow]

New tires and low weight rims and/or moon discs. Pump up tires to sidewall max. (if you think tires can take it.) larger diameter tires? There should be room and it may be cheaper and easier then changing out the read differential.

The simple things may help the most compared to the effort invested:

1) Weight reduction - spare tire in back. Speakers may also be very heavy.
2) General maintenance - Oil change. Wheel bearings. Transmission fluid. How are the shocks doing? Are the tires/wheels balanced? When was the last time spark plugs were changed? Oxygen sensor?
3) See where the engine runs leanest (if possible) and lowest speed overdrive will engage (40 mph-ish?).
4) Engine braking on downhills where coasting not possible. Does this have a carburetor or is it fuel injected yet?

I think about 25 mpg should be quite possible, despite the weight and vehicle age. Good luck!