Question

I have a 2005 GMC Sierra Z-71 half ton crew cab short bed and the other day I tried putting it in 4x4 low and it wouldn''t go it would go into 4x4 hi, then when I was done I put it back into 2wd everything seemed to work until I was about five miles down the road and while driving 55mph the truck seemed to go into neutral, the gear shifter on the column was still in overdrive and the push button 4wd selector showed it was in 2wd, so I stopped on the side of the road and the only way I could get it to move was to put it in 4wd hi and it moved and felt like it was in 4wd hi so then I put it back in 2wd and it seemed like it was in 2wd, tried it in drive and reverse everything seemed ok so I continued down the road and stopped to get gas. When I started the truck and put it into drive it would not go again I had to put it in the Auto 4wd mode to drive it home everything sounds o.k. but it acts like it is just not fully engaging in 2wd mode and it still will not go into 4wd lo?