driving the truck i hear a bang truck wont go into gear in 2HI so i put it in 4 auto. next day driving it around 35 mph it locked up. wouldnt go into forward or reverse for less then a foot without locking up. dont think its the tranny. possibly transfer case? never heard of this happening before. its a 2001 chevy silverado ls 4x4 1500. any input or recommendations would be helpful thanx