4 wheel drive Problems

I have a 1997 S-10 Blazer that seems to shift into four wheel hi and shifts into four wheel low but the front axle doesn't engage. The lights stay on but the axle doesn't engage.Reading other posts can it be the vaccum switch and where is it located, some I guess are on the fender well and others on top of the transfer case.Possibly a bad vaccum hose. Thanks Gordie at [email protected]