2004 Ford F150 wheel drive

I have my truck in 2L and drive and it seems to slip out ,but not all the way and makes a whining noise while driving. I then try to put it back and it keeps whining. I put it in 2H or any other 4 wheel drive for a minute and then put it back and 95% of the time it's ok. Also, if I hit a big bump it seems to do the same thing all over again