I am new to the forum but bought my truck to tow my Jeep(s). The new unit hubs that are used, allowing the front end to turn all the time, do not have a noticable HP drag. This design has been used for 10+ years on Jeeps and none of the kits to install locking hubs affect fuel economy. The locking hub kits are used to allow driving the Jeep after you break the axle, and to upgrade the axles to Chrom-moly axles. On our trucks it would be interesting if there was a noticable fuel difference, but I am sleptical that there is. I actually bought my 04. 5 truck because the front axle is a unit hub design, fewer problems, no maintance except to change the diff oil and grease the ball joints.
Steve