change your fuel filter...if it is anything like a 22re it is a PITA to get too, but it would probably really help (especially if you don't know how many miles are on it). I changed the filter on a 97 Tahoe and...believe it or not...it went up 2 mpg on the highway.
U think the fuel filter would make that big of a difference? there is a pre filter in the tank too so i would think the fuel going thru the line filter would be pretty clean