gene i had a similar issue recently after changing the transfer box, i could get high and neutral but no lowbox, it turn out the splined ring on the side of that housing was put back on the wrong orientation, first i tried moving it back and forth one spline but that gave me low/ neutral but now high then.
have a look at a picture in the haynes manual and check it looks the same, the linkage can be on the wrong way if that makes sense