1986 Burb issues with 4x4

I am having issues getting my 4x4 to engage. I shift the 4x4 into 4H and i move forward and it feels like it is grinding. It does not engage at all though. I take it back out and back it up and the 2WD works just like normal. It does this in both 4h and 4L. I don't use the 4x4 much, usually mud roads while hunting pheasants, but it just up and won't work now. I want to make sure I am not damaging the 4x4 by trying to get it to work.

Any suggestions would be great, we got 8 inches of snow to the north of us yesterday and winter is bearing down on us.

Thanks!