This issue is pretty common, actually. Check your linkage, as suggested above. If that doesn't take care of the issue (it probably won't) then you'll have to replace the synchronizer assembly inside the transfer case. A full rebuild kit doesn't cost much (less than $300) and if you have access to a lift, the rebuild can take less than two hours start to finish. One thing to look out for though, is that there is a ball and spring that is used to hold the shift lever in position- that spring wears out, and isn't typically included in the rebuild kit.
When you cycle the transfer case shifter from 2 hi to Neutral, and then into 4 Hi, do you feel the handle 'lock' into position at each point? or does it freely slide between the settings? If the latter, your spring is worn out, and maybe even broken.
The spring and ball are part #70 & 71 on the parts diagram, and interface with part #55:
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