21 MPG hwy should be possible on that truck, but like everyone else has already said, there's just so many variables that will affect it (what axle ratio it has, and your driving style being the biggest two factors).
My '94 4.0L on 33s gets 21, on rare occasion, 22 MPG doing 60 MPH.
Around town (mixed) mileage averages between 17-18. I have 4.10 gears which is a slight bit under what's ideal for 33s.
Pumping the tires up real hard (35 PSI) will improve MPG at some expense of ride quality, and evenness of tread wear (though if you wheel it aired down a lot, it can help even the treadwear out a bit... or so has been the case on my BII anyway).