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Cleaners/ flushes are notorious for causing more problems than they fix. sounds like time to pull the injectors. nothing replaces inspection/repair and replacement.
Should get in the neighborhood of 12-15 with it depending on loaded or not and how fast its driven.
They guy with the 94 3/4ton wasn't exactly a stickler for maintenance but, the guy with the 96 1/2 ton fought it for a long time before trading it on a newer truck. I was driving an 84 with a windsor at the time and got twice the mileage they did.
Is the CEL on right now? what code are you getting? I maybe wrong but w/ auto hubs the4x4 may not engage until wheel slip is detected. And please, DONT run anymore cleaners thru your engine, it sounds like your injectors are gummed up probably because of the fuel sys. cleaner. after all the crap has to go somewhere and injectors are the smallest hole. Invest in new injectors, take your time and do a COMPLETE tune up. this should turn into a great rig w/ a little work.
Another way to test injectors is to take it to a mechanic or dealership, even though it's not OBD II they can test the injectors and most reputable mechanics have a REAL injector cleaning system. I've only seen my neighbor do the injector test on late model stuff, but it will actually show the amounts each injector is passing, then he uses the injector cleaner which actually bypass the gas and the vehicle will run this solvent through. I asked him about doing the injector test on my 95 which is OBD I and he said it would work mostly but not as much information could be gained.