Converting to lock out hubs was something that existed for older 4x4s usually equipped with full time four wheel drive. The lock out hubs was part of a kit to convert the truck to part time four wheel drive and included a part that needed to be installed in the transfer case (the transfer case in question was the full time NP203) However converting the transfer case from full time to part time did not result in huge fuel economy gains. Usually the gain was about 1 or 2 mpg, and sometimes there wasn't any gain at all.
As far as I can tell (and I've looked) there aren't lock out hubs available for the Dakota, not that it would matter because the majority of Dakotas are equipped with part time four wheel drive anyway. Those equipped with "all" wheel drive can be shifted into two wheel drive. Even if it were possible to install lock out hubs or modify the hubs so that they can be disengaged, you wouldn't realize much mpg increases because the front axle assy is already very efficient as it is.