4WD is not the same as AWD(all wheel drive)
4WD actually connects both front and rear axles together, giving both axles an even 50/50 split of the engines power
AWD uses a fluid coupler between the front and rear axles, giving a 30/70 split of power

So you shouldn't use 4WD on good traction surfaces, i.e. pavement, wet or dry, it doesn't allow for slipping between front and rear axles
Gravel, dirt, snow or ice is when you can use 4WD

AWD can be used all the time because it has the fluid coupler that slips all the time, but that also makes it less than ideal for off road use


OK, that being said it reads like the Computer is not getting the message that you are in 4WD or you have a vacuum leak
Do you have a manual or Automatic transmission?

And do you have electric shift 4WD or Manual shift(stick shift on the floor)?

I will assume automatic trans with electric shift.

2000 Ranger 4WD was the last year of the GEM module for 4WD control and also the last year for Pulse Vacuum Hubs
I would suspect the problem is the Pulse Vacuum Hubs(PVH), there is a Vacuum leak in that system which is causing the engine to run rough when its activated, 4WD activates it

PVH uses engine vacuum to automatically lock the front hubs to their axles when 4WD is selected.
Ford only used this system for 3 years(1998-2000) because of problems.

Diagram of PVH vacuum system seen here: https://therangerstation.com/forums/attachment.php?attachmentid=23646&stc=1&d=15262510 09

I would locate the PVH vacuum hose at one of the front wheels, then follow it to the hose splitter(T), then follow the main hose back up to the PVH Solenoid, check for breaks or cracks in the hoses.
There will be two hoses on that solenoid, the one you just followed up to the solenoid and the Vacuum Source hose
Unhook the Vacuum source hose and PLUG IT, the hose
Start engine and shift into 4WD, don't have to move the truck, just shift it into 4WD and see if engine stays running smooth, REV it up a few times or drive it if you want just to see, front axles won't be engaged, if engine runs same as 2WD then yes the PVH system is the cause of the poor idle and almost stalling.

The problem could just be a bad vacuum hose from the solenoid to front wheels, check those 3 hoses and the "T"
BUT...........it could be a bad solenoid or a bad vacuum hub, and because of the unreliability of this system, you don't want to replace these.

There are two options, both would mean you will leave that vacuum source hose PLUGGED
Change the PVH hubs inside so they are LOCKED all the time, all 2001 and up Rangers were this way, Ford called it "Live Axle" front wheels were always locked to axle, 4WD was done only in transfer case
How to lock PVH seen here: https://therangerstation.com/Magazine/Apr04/offroad.htm
You can still use "shift on the fly" with hubs locked full time, same as all 2001 and up Rangers can

OR

Get manual hubs to replace PVH, seen here: https://therangerstation.com/tech_li...hub_swap.shtml
These allow you to lock or unlock the wheels manually
You can just leave them locked through the winter months and still use "shift on the fly" 4WD, no difference, then unlock through the summer