Not sure if your jeep has the vacuum front axle disconnect, but if it does that might be the problem.
When you go to 4WD low range, are you sure you have 4WD?
The plastic multi-colored vacuum lines they use in this system are impervious to all known automotive fluids, with the one exception of battery acid.
Curiously AMC decided to run these lines (in fact mounted some system components too) right under the battery tray.
If any of these lines appear to be cracked, then they are sucking air. Not a good thing in any vacuum system.
You can google "Posi-lok" if you want the cable deal (I chose this), or you can re-do the lines (cheaper).
If you go with the re-do, here is a diagram;
The storage tank looks like a black plastic baseball, & the control switch is on top of the transfer case.
If you don't have the vacuum disco, someone else will need to help.:-|
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