1996 dodge 3500 12 valve 4x4 1st gear shifts great as does second gear but

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But when I shift into 3rd gear there is a loud knock or bang. Not sure if it's the transmission or the transfer case. Please help if you know what the problem. I hope it's a easy fix. It shifts in all 5 gears but when I am driving and try to shift into 3rd gear it slams hard.