It does not matter what the t-case ratio is in respect to the axle ratios, thats just where you get another reduction from "low range". In high range the t-case is a 1:1 ratio, meaning the input speed is the same as the output speed, in low range the input speed is 2.72 times faster than the output speed.
If it grinds in 4low you either have something broken inside the t-case or the shift linkage is out of adjustment and its just not getting fully engaged into 4low. I would want to assume the shift linkage is out of adjusment, there is an adjustment rod/pinch bolt under the Jeep infront of the t-case right beside the trans on the drivers side. I would loosen that bolt and slide the linkage a little bit to see if that helps.