Here is my plan of attack of stuff that I have used and know works.
First head to a Detroit garage and have the charge air cooler tested, and make sure you don't have any other mechanical problems that hurt fuel mileage. Since you just had an in frame I am guessing your ok if it was done right. Then you can start modifying things.
Slow down. Depending on which gear ratio you have: 3.70 run 57, 3.55 run 62.
Pittsburgh power flow through muffler or Walker Mega flow muffler
Fleet air filters
OPS and switch to synthetic (the synthetic oil helps with fuel mileage)
FASS fuel system
Low rolling resistance tires, also make sure they are centered and balanced. Align the truck also if you are experiencing wear on your current tires
De EGR the engine. Then you can add Pittsburgh powers manifold and turbo and have them program your ECM
Now if you want to start really modifying the truck you can change the exhaust system to take the mufflers off the front of the truck and make either a weed burner if you don't idle or put the mufflers up the back behind the truck. This will reduce wind drag.
You can also take the dual air cleaners off the outside and get an under hood filter. This will reduce wind drag.
I have not tried this yet but I believe they work, there are several people here that have done this.
Micro blue bearings, tranny, and rear ends