You have a Borg Warner 1354 transfer case(BW1354) electric shift, there is also a manual model, and no they can not be converted.
I doubt it is the problem, and there is no "grease" in them, they use ATF, yes automatic transmission fluid, Ford does make a "special" Transfer Case ATF for them, up to you on whether or not you use it

In 2008 there are only 3 parts to the 4WD system
Dash buttons or ****
4x4 Shift control module, it has the relay
Electric shift motor

Button/**** connects to Module, it has 3 resistors inside, module sends the button/**** a fixed voltage(like 5 volts), RETURN voltage tells the controller what position the button/**** is in, 2WD, 4hi, or 4low.
Button/**** RARELY fails, very very last thing to check

The Shift modules were known to fail, 2006 to 2012 Ranger used the same module, they seemed to last longer than the 2001-2005 modules

Electric shift motors usually failed from disuse, the brushes and rotor in the electric motor part would get corroded from sitting and lose contact, best practice is to shift to 4hi, then 4low, then back to 2WD once a month just sitting in the driveway, don't have to drive it, just shifting cleans the brushes and rotor

There should be a 20amp fuse in the engine fuse box that powers the shift motor

I don't have a 2008 diagram, but get owners manual here and it should be listed: https://www.ranger-forums.com/genera...1-models-3747/

 

 

I would pull off the motor and test it, and also see if shift rod moves easily

Pictures here: https://www.explorerforum.com/forums...-motor.123542/

In the first and second pictures you can see the 2 wires that run to the Motor part of this unit, yellow and orange wires
If you give these wires 12v and Ground motor should turn, then REVERSE 12v and Ground, and motor will turn the other way, there is a stop inside on the contact wheel, so motor will only turn so far one way and the other way.
The wires colors may not be the same but location on motor will be

You can remove the motor and plug it back in to the harness, then operate the dash **** to see if motor is moving, it should move to 4high and 4low easily and back to 2WD, and dash lights should confirm each movement, that would mean the ****, 4x4 module and shift motor are all OK, and problem may be transfer case, BUT......................
Electric motors can get weak, so even though they work with no load doesn't mean they can turn the shift rod, so test shift rod movement

3rd and 4th pictures show the shift rod, the part the motor turns
In 4th picture it is pointed at 2H(2WD)
From 2H it can ONLY TURN counter clockwise to 4H(high), and then counter clockwise to 4L(low)
Between 4H and 4L is NEUTRAL, directly across from 2H, although it is not marked, so VERY IMPORTANT to have a wheel BLOCKED, because it WILL ROLL AWAY when you shift transfer case to NEUTRAL!!!!!

Rotate shift rod clockwise from 4L back to 2H

You may need to rock the truck forward and back to shift transfer case, that is normal, the gears inside attached to rear drive shaft have to line up

The contact wheel and reduction gear doesn't need to be removed unless you want to rebuild the motor, up to you, but mark these before disassembly

The shift motor works just like a power window motor, +/- on its two wires rolls window up, and -/+ rolls window down

The 4x4 module has 2 relays inside, one for each motor wire, when relays are off BOTH motor wires are Grounded(-)
So the 4x4 module just needs to activate 1 relay, which connects that 1 motor wire to 12v(+) and motor should start to turn because other wire is already a Ground(-)
Which relay it actives determines the clockwise or counter clockwise movement of shift rod
The contact wheel "tells" the 4x4 module what position the Shift rod is in, 2H, 4H or 4L, so its important this is aligned correctly
If the 4x4 module activates a relay and it doesn't get instant feedback that contact wheel is turning(contacts open and close) it will deactivate relay and Flash 4WD lights on the dash to indicate a problem
The 4WD lights should come on when key is turned on, thats a BULB TEST not a 4WD test, just FYI, so if 4WD lights don't come on with key on then bulbs are burned out.