IMO the negligible gain that you would get is not offset by the fact that's its a complete PIA to do the conversion. Not to mention the costs of the hubs.

I used to have a Dodge 4WD pickup with manual locking hubs on the front axle. I didn't run extensive testing, but on those occasions when I did check I could measure no difference in fuel mileage with the hubs locked or unlocked. Leaving them locked kept the front axle oil seals from drying out and made use of 4WD much easier, because I didn't have to get out and lock the hubs.

I don't know about the gas engines, but with the Dodge Cummins diesel truck, people are seeing between .5 and 1.5 mpg gains. Yes, they are a pain to install, they are expensive but you get added fuel economy and they are cheaper to rebuild than replace worn out hubs. With that said, you can replace quite a few hubs for the cost of a spin free hub assembly.

You also need to check your o2's and ensure that you're not running rich. I had a 1998 Z71 4x4 5.7L ext cab and I got 15 no matter how hard I hammered on the gas. 16.5 highway at 75mph.

That was with 33's too. Not stockers.