Last week I bought a used 99 four-wheel-drive ranger. I just needed something to tool around town in. Took it out on a back road and found a little bit of mud to test it out in. It was nothing serious and I couldn't have possibly gotten stuck but when I turned the switch to four high (waited for it to engage) and went through the mud it was very obvious that the front tires were not pulling at all. Only the rear wheels were driving.

I started to get concerned thinking that maybe the four-wheel-drive wasn't working. But then I tried to go up a small hill and immediately the back wheels started spinning because the incline was too steep. I switched it to four high and it crawled up the hill without a hitch.

So my question: can you help me understand why the front wheels weren't engaging in the mud but they did when I climbed the hill.

 

The 98 and 99 4WD Rangers had a vacuum actuated hub locking system. They tended to develop leaks in the vacuum system and are not very reliable. You may not have gotten hub engagement when you went through the mud but did when you climbed the hill.
There is a manual locking hub kit that can be installed. I think they are still available.

 

Welcome to the forum

Yes +1 to what IN2 FX4 said

1998 to 2000 Rangers used PVH(pulse vacuum hubs) which worked but not for very long, so Ford abandoned them as early as 1999 on some Ranger models, in 2001 all Rangers had Live axles, hubs just stayed locked all the time.

You could spend time and money on fixing the PVH system, but it will just not work again at some point when you really do need 4WD.
You don't know it is not working until you need it, lol.

Best bet is to use AVM/Rugged Ridge Manual hubs, 100% reliable
Transfer case shifting has nothing to do with the PVH system, totally separate systems
(just a heads up about the electric shift transfer case, once a month shift it from 2WD to 4hi, and then 4lo and then back to 2WD, you don't have to move the truck, just shift transfer case, this keeps the electric motor's brushes clean and shifter parts lubed)

How to for manual hubs here: 1998-2000 Ford Ranger Rugged Ridge (AVM) Locking Hubs


There is also a way to lock the hubs full time with PVH parts, seen here: Permanently Lock Your 1998-2000 Ford Ranger PVH Pulse Vacuum Hubs

I have never tried this but it seems to work fine
And no it is not an issue to have the hubs locked all the time, 2001 and up Rangers have that.
It is reported MPG drop is around .3 MPG so not 0 but not much either
And AWD vehicles operate this way as well