4WD not staying engaged

I got vacuum hubs like most other 1998-2000 rangers the 4wd would not kick in. Replaced the fuse for it and started working fine. Then comes winter time and I start spinning on snow good all terrain tires to. But recently found out that when I first switch to 4wd my 4wd will work fine no spinning at all and then like 10 to 30 seconds later it will kick out of 4wd and go back to 2wd without me touching anything. Losing pressure through a slow leak I think though i'm not sure.