truck wont go into 4 low
 

i have a 1998 chevy silverado, with push button 4 wheel drive and my truck will go into 4 hi fine without any problems but when i try to put it in 4 low it wont go in and it just grinds terrible. I have my truck in neutral and once i put it in gear it starts grinding until i put it in 4 high again. any ideas what is wrong with my truck. thanks