1999 was 9th year of 4.0l OHV engine in Rangers/B4000s good reliable engine if NEVER overheated, these will crack a head if overheat even once, so "heads" up, if hose breaks or coolant gets low DO NOT try to "make it home", pull over and let engine cool off, add more coolant and drive it a short distance if you need to but pull over again and let it cool off.
If it takes you 3 hours to get home doing this you are making $300 an hour, because it can cost $900 to redo the heads, when a broken hose may cost $10 to replace.
'99 has the Pulse Vacuum hub(PVH) 4x4 system, it was unreliable after a few years old.
Ford/Mazda switched to "live axle" on all models in 2001 but some models were "live axle" as early as late '99.
Live axle means there are no lock/unlock options for the front wheels, they are always locked and front axles and drive shaft always turn when vehicle is in 2WD.
It was found that it only decreased MPG by .3-.4MPG
If you have PVH system and have any problems with it you can lock the wheels permanently making it a live axle setup for free.
There is also a company that makes manual hub converters for the PVH system
On all electric shift transfer cases you should shift transfer case into 4WD at least once a month, you don't need to drive it, just shift it to 4WD and then back to 2WD.
The primary failure in this type of shifting system is the electric motor and it's brushes.
The brushes apply the power to the part of the motor that spins, they are pushed against this spinning part by springs.
If motor isn't used for months and months the brushes can get stuck or contact between brushes and armature can get intermittent, so when you do try to shift into 4WD you get a Flashing 4x4 light, meaning no shift happened.
Often if you get under the truck and tap on the shift motor a few times then try to shift again it will start working again, but you would need to remove and clean the motor to make it reliable again.
Shifting it once a month keeps brushes loose and contacts clean, it doesn't mean it will never break just makes it reliable longer, lol.