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On most automatic hubs you have to back up as much as 75ft to unlock the front hubs.
Pulse Vacuum Hubs(PVH) were only used for a short time, reason being, too many service repairs under warranty, 1998-2000.
Ford switched to "Live Axle" as early as 1999 on some Rangers, official switch was 2001 model year, so I assume they knew the system would be trouble prone even before the first 1998 Ranger rolled out.
Have a read here on troubleshooting the PVH system: Ford Ranger How To: Troubleshoot/Fix Your PVH Hubs (1998-2000)
And know going in that any fix is temporary with PVH.
Best fix is switching to manual hubs: 1998-2000 Ford Ranger Rugged Ridge (AVM) Locking Hubs
Or locking both front hubs permanently, "Live Axle" : Permanently Lock Your 1998-2000 Ford Ranger PVH Pulse Vacuum Hubs
Make your 1999 into a 2001, lol.