I have a1998 ford f150 and I can't get the four wheel drive to lock in I put a new motor on it which the old one was bad and all it wont switch to four hi or low from the range selector but if you go ahead and put it in the range you want and then unhook the battery terminal it will lock the front end in and the light will come on but the transfer won't lock in and it won't switch back out until you take the battery terminal off again

 

You've got a ground that's bad or a wire that got pinched during the motor swap.