I would almost guaranty its the front output shaft. Mine did same thing and what is happening is the shift collar that connects the front main shaft to the front output shaft is not engaging the front output shaft and so the front output shaft needs to be shimmed. What I did was take the front section of the case off (where the shifter goes into the case) and shim the front output shaft between the bearing and shaft ~ .08-.10 inches. Also if you put your case in 2wd the detent ball will go back in its hole, then you can pull the 4wd shifter fork out and flip the shift collar around so you will have fresh teeth to engage the front output shaft.
Have you installed a HD front output shaft?