with transfer case in netural and engine running I can change thru all gears ok no cruching etc but when I engage the transfer case it crunches/grinds into gear

have had the transmisson out but hav"nt pulled it apart except for removing the top to remove it, have checked the forks are in the right places and changed the oil after a swim but drove onto the trailer that day
it is a 3 spd t-90c box with a t 18 transfer case in a cj4a jeep
ideas cos I'm fresh out of them
Warren