I found my 4WD shifting to be intermittent. About the only time I ever shifted into it was a few times during the winter. Now I make it a practice to shift it into 4 once a month and I have had no problems with it since. I don't know if it was the electrical part or the manual but something must get some corrosion on it or get mucky if you don't shift it in for a long time. My 2 cents.

1997 T-100 4X4 Xcab,Warn Hubs
02 Camry LE
2008 Yaris