2nd Gen Non-Engine/Transmission No 4H, but 4L works

[TWirth]

This is a '96 Ram 3500 5 speed, 4x4, 100K miles, pull it into 4H, you can feel it contacting something that feels like a gear that isn't engaging, 4H does not engage, but 4L works. It seems to run fine in 2H, and 4L. Would it be a problem in the shift linkage? Is it possible that it is internal, in the transfer case? If it is, can I get by with using it that way for a while, just changing the oil? Any guesses what parts it would need if it is in the transfer case?