855 with a hard to disengage 4 wheel drive

There are times when I have real difficulty disengaging the four wheel drive lever. At other times it will be fine. Appears to be no problems when engaging the drive lever, just when I try to take it out of four wheel drive. Anyone have any suggestions about this. I should ad that I':hi:ve tried disengaging while moving forward very slow & sometimes that helps. Sometimes at a stand still I can get it out. But it can be a real pain.
Thanks-Maddog:hi: