Your shifter should be forward and left in 2WD.
4WD High is straight back and to get to 4WD Low you have to move the lever back through neutral (centered) before going into 4WD Low which is forward and right.
The most common cause for the D20 walking out of gear in 2WD is worn or broken shift lever torsion spring.
From your description of the symptoms, the shift lever isn't moving when you experience this issue. When the TC walks out of gear in 2WD, the mainshaft slider gear disengages from the transmission output shaft gear and the TC is in neutral. This was a common problem with the D20 in the Cherokees and J-Trucks with the single lever.
If I am correct and your torsion spring is borked, the challenge will be to find a replacement...
Here's the exploded view of the shifter from the TSM:
Oh, and you might want to check to see if the shifter walks out of 4WD High or Low while you're at it. If it does, you're looking at opening up the TC to fix stuff. Not a difficult thing, but messy with the TC still in the truck.
If you need more help with this, be sure to shout out.