Sounds like your Xfer case is coming out of 4-wheel drive but your hubs aren't unlocking. This happens especially if the 4x4 isn't used very often. Mine seem to unlock better when I'm moving faster - how fast were you going when you put the lever to 2H?

When mine won't unlock going forwards I just put it in reverse and back up for about 10 ft. This has always worked for me, but the way fasteddy describes might work even better, so I'd try both ways.

If that doesn't do it then you probably have an electrical or vacuum issue <img src="/forums/images/graemlins/scared.gif" alt="" />, so hope that backing up does the trick.