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I wouldn't worry about lockout hubs. The mass of the CV shafts is negligible compared to the tires and other driveline parts. The way the front diff is designed the pass axle is 2 parts, and uses a vacuum solenoid to engage the pass side to the diff, which is spinning in conjunction with the driver side. Yes, 25lbs of metal are spinning at all times. But when people swap over to the Bravada pass. axle, which is one piece, leaving the whole assembly rotating, and see no drop in mpg, I'm inclined to believe that the full weight of the truck (4000lbs) doesn't care about 25lbs rotating under.
At $430 a hub, It's not worth it. For twice that you could swap a 6.2D or 6.5TD from a FS chevy in and double the mileage. I'd like to use a Cummins QSB3.3T personally, but that requires a solid front axle to support the 780lb motor. But with 30+mpg with hotrodding, who knows. Finding a lightweight diesel that makes as much or slightly more power than the 4.3 is challenging.
/tangent.
I think you're just stuck. The 4.3 is thirsty, the driveline not the most efficient, and it's a heavy truck. Amazing that a FS can get better mileage and weigh 1500lbs more. Short of heavy mechanics there's not much you can do to improve mileage.
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