4x4 low does not want to work !!!!!!!!

[strokit0919]

i got a 1999 f.250 7.3 and to put it in 4x4 i only have to lock 1 hub in! if i even touch the other its a ***** to get it to go in to 4x4. when i try to put it in low it grinds then go into 4x4high then it will start grinding again and it does this for about 4 till i put the switch into high again! and sometimes it goes n 4x4low easy as **** but it pops out and then its a ***** to get into 2wd or even 4wd. wjat could it be need to fine out. [SIZE="5"]