90 4Runner 4x4 low range problem

One of the gear selections in the Transfer Case is Neutral. From 2HI, you select 4HI, then N, then 4LO. When shifting out of 4LO, you pass through N before you get to 4HI.

You are describing a problem with the shift linkage for the transfer case shifter. That's where I'd be investigating. I suggest you need a helper that can move the lever for you as you observe the movement of the rods and bars. I suspect you will find a loose set screw or bolt on one of these rods.

I further suggest that revving the motor is not a cure. You should try over shifitng towards 2HI to see if you can get the transfer case to jump past the N position.