Grinding when trying to shift out of 4L

[Chrispbrown36]

My 87 did great in the snow today, until I pulled into the driveway. We have a decent hill at the beginning of our driveway so I put it down in 4L to get up it. When I got to where I wanted to be I put the tranny in neutral and tried to shift out of 4L and it started grinding. I let it roll a little then tried again same thing. I pulled forward a little same thing. Can't figure out why it wouldn't let me shift out of 4L. Could it be the incline?