Wheel Lock

[FMS88]

The lock hub transfers power from the axle/drive shaft to the wheel hub. So without the lock hub, I wouldn't expect the wheel to rotate when in 4wd on the lift. Either someone's improvised a repair to keep it effectively locked in 2H and 4H, or there's enough dirt and gunk in there to achieve a connection that's rotating the wheel. If it is somehow locked, it might cause the other auto lock hub to lock and unlock even when not in 4wd, but it's unlikely that will damage that hub or the rest of the front drive train - just add a little wear and tear and increase gas consumption as you suggest.

My biggest concern is for the needle bearing inside the steering knuckle. It supports the axle shaft where it enters the knuckle. Without the lock hub, water and dirt can easily penetrate and destroy the needle bearing and the axle's bearing surface. The lock hub also helps protects the wheel bearings so check those out, too.

Choosing between manual and auto hubs depends on your preference (staying in the cab or getting out to switch the hubs). The manual hubs have fewer parts so their easier to maintain, but maintenance is not a frequent concern. Installation of both is the same requiring a 10mm allen socket and snap ring pliers. A dial gauge to check/adjust the axle shaft end play is recommended especially if the lock hub is new.

As to which hubs fit, any manual or auto hub for a first or second generation pickup will work. I think the same applies to hubs from first and second gen monteros and raiders.