Hi,
The idea that it won't turn the front driveshaft is interesting. Please confirm something for me. Is it the NV (not ND) 231?
Also, this transmission should have a manual-shift transfer case. The 4wd is engaged via the shift lever, shift fork, and the shift lever in the truck. I don't suspect there is internal damage, or you would be hearing noise. Low fluid, although not a good idea, shouldn't cause this.
There is an adjuster on the shift linkage. Are you certain the bolt that sets it after adjustment isn't loose or allowing the linkage not to fully move the shift lever on the transfer case? Have someone shift the transfer case through the different selections while you watch to see if there is an issue with the adjuster. If the bolt is loose, it will allow the shift rod to extend and retract, so it may not fully be going into 4wd. (See pic 1)
Also, the transfer case itself doesn't rely on vacuum.
Let me know what you find.
Joe
See pic below.
Image (Click to make bigger)
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Thursday, January 13th, 2022 AT 7:47 PM