Hard to say from such little information. I recall years ago that my brother's Toyota did something similar when the clutch was nearly worn out. In high range (either 2H or 4H), the clutch would slip so bad that it was nearly undriveable in high. The clutch didn't slip in low, but, obviously, there's no speed in low. Could your's be doing the same thing? The clutch so badly worn that it will only go in low?