If your front Dshaft is not turning, I'd check the transfer case linkage for proper operation. If that's ok, then it might be shift forks inside the trans case.
If your front Dshaft is turning, then it's probably the CAD, a fairly common prob. I don't know how 4hi and 4low would affect the CAD, but maybe there's a switch for it that is only getting activated in 4hi?