This FAQ was written by Warwagon
All Dakotas and many R/C's and 1/2 tons (some 3/4's I think too) have this system and I went through this last winter with my '88 dak - "The Harlet" - No 4x in either HI or LO. The front axle simply would not pull regardless of the transfer case mode used.
If the front driveshaft doesn't spin in sinc with the rear in 4x Hi or Lo the trouble is in your T/C. If the front shaft does spin at the same rate as your rear shaft, then the front axle isn't engaging (like mine behaved) . There are 4 possible trouble spots to look at, I'll take them in order of ease of fix and cost:
1) You may have a vaccume hose that is cut or chafed and has a leak OR a vaccume plug may be off. Visual inspection is easy on this. The lines on my '88 were still soft and pliable. Dodge did a good job of keeping them up out of the way and anchoring them so physical damage to them is unlikely, although they are a bit of a PITA to see in places. Also, sometimes water gets in these hoses if the breather is broken or missing and will freeze in there when below 32F, effectively cutting off all vaccume signals.
2) Your vaccume resivour ball may be leaking due to a crack or other damage. This thing looks like a bocci ball with vaccume lines on it. To test, draw a vaccume on it, if it holds for a minute or so it should be functional. This is an unlikely break unless your rig took an undercarrage hit from an obstacle or a BF'n rock nailed it.
3) The vaccume switch on the T/C may be galled or shot. There is a vaccume plug with 4 lines going into the top of it on the rear of your T/C, attached to the base of the extension housing on the drivers side. It looks like an elongated nut that screws into your T/C rear extension housing. The 4 lines are for: A) vaccume source, B) vent, C) axle engage and D) axle disengage. Trace the lines to see which is which - the axle engage/disengage are bundled and go ... you guessed it ... to the front axle. Pull the vaccume plug and unscrew the swich from the housing. There is a ball on the end of it that sometimes gets galled. clean it, free it's motion and retry it. If it still doesen't function, pull it again and place a vaccume source on the source fitting, you should have vaccume on the axle disengage port, then push the ball in and you should have vaccume on the axle engage port. If the vaccume switch doesn't act this way, it's shot. This is what was wrong with mine and a new one from Mopar cost me about $17 (if memory serves). It is also THE MOST COMMON PROBLEM with failure to engage the front axle on the Ramtrack (Dodge's term for central axle disconnect) system. At such a low price, it may be wise to replace it anyway as many origionals fail at some point.
4) the vaccume shift motor on the front axle or its shift fork may not be coupling the axle halves. To check this, pull the 2 port vaccume connector. Then pull a vaccume on one of them The axle halves should be either free or locked. Then pull a vaccume on the other one. The reverse action should happen. If it works like this, then your problem lies with items 1-3. If it does not engage/disengage the axle halves then you need to pull the shifter motor to find out why. Remove the bolts that hold the shift motor plate on the axle. Slide the shift motor out. Again pull a vaccume on it's 2 ports. The shift fork should slide back and forth as this is done in succession. If not, the diaphragm is most likely toast in the shift motor. I've never had to replace this item, but I've heard it costs around $100 . Just be sure the axle coupling sleeve slides back and forth on the axle shaft splines and that the shift fork engages the groove on it when you do the install. Use a new gasket or RTV when you put it back on as you don't want water and grit in your axle.
PM - Now would be a great time to check the levels/condition of lubes in your T/C and axles too! The front axle shift collar sometimes gets galled if you've pulled water into your front axle . Dodge recommends Dexron II in the T/C and don't forget friction modifier in the rear if you have a Sure-Grip limited slip (optional equipment) and you change the gear lube. Some gals (and guys) are switching from dino-oil to synthetic lubes for these parts in the hopes of reduced wear and increased mileage. You may want to consider this.
ALWAYS TRY THE FREE OR CHEAP STUFF FIRST!!
Best of luck in your quest for front drive traction-action .