I have a 2000 XLT, manual, 4x4. When I go to 4x4 Hi the shift motor engages, and the front driveshaft locks in so it will no longer turn freely by hand. I have vacuum to the switch under the air box, so is it safe to assume one or both hubs isn't engaging, or am I missing something? I bypassed the vacuum switch and still had no 4x4.

 

Welcome to the forum

All 4x4s, not just Rangers, have an OPEN front differential, this means BOTH hubs need to be locked or no 4WD
The 1998-2000 Rangers used PVH(pulse vacuum hubs)
Ford realized these were a bad idea pretty quick, lol, some 4.0l Ranger 4x4s were getting Live Axles as early as 1999

With transfer case in 2H, 2WD
Start engine and by pass the Vacuum solenoid to send vacuum to both hubs
See if you can turn the driveshaft
It shouldn't turn if both hubs are locked
If you can turn driveshaft then see if you can tell which axle is turning, that is the hub that's not working

The more common fix is to install AVM/mile marker manual hubs and get rid of the PVH setup

There is this "fix": https://www.therangerstation.com/Mag.../offroad.shtml

Never had a PVH Ranger so never done it my self
Full time locked hubs are what all Ranger had from 2001-2011, called Live Axles

Manual hub conversion seen here: https://www.therangerstation.com/tec...hub_swap.shtml