In 2wd I have ZERO slack in my drive train, but in 4wd it feels like I have slack in the drive train. If I'm driving down the road and let of the gas and then press the gas I hear and feel a pretty good pop/clunk as if there is slack in the front drivetrain. But it only does it in 4wd.

Is there a specific problem that causes this or is it normal. Also I did do a search on it but all the threads where talking about slack all the time.

 

Do your u-joints have play in them if you grab 'em and shake 'em. Have you regularly greased the zerk fittings on your drivetrain? How do your CV axles look?