Look here: Permanently Lock Your 1998-2000 Ford Ranger PVH Pulse Vacuum Hubs
Is this what your front hubs look like?
If so you have PVH(pulse vacuum hubs) and can get AVM manual hubs that will work
Look here: 1998-2000 Ford Ranger Rugged Ridge (AVM) Locking Hubs
If you don't have PVH hubs then you have "Live Axle" which means front axles are always locked and 2WD or 4WD is selected in transfer case only.
There is no manual hub conversion for Live Axle
BUT(big but).............you can convert Live Axle to PVH parts and use AVM manual hubs.
Conversion article here: How To: AVM Hub Conversion, 2006 Ranger - Ford Explorer and Ranger Forums "Serious Explorations"®
This is purely a users choice, Live Axle only loses .3MPG so not a real drag on the pocket book for having 4WD available.
People after high MPG wouldn't get a pickup truck in the first place, aerodynamics of a brick, or a 4WD pickup truck for sure, lol.
I like the idea of Live Axle because most 4WD Rangers don't use the 4WD that often, this means when "unlocked" the u-joints in axles and drive line sit in one position and differential parts and axle seals don't stay lubed.
So when 4WD is used everything starts up dry for the most part.
With live axle all the parts stay moving and lubed.