[Jonboy]

I haven't had a 4x4 in over 15 years, so this is out of memory, but it seems like the trans needs to be in neutral to go between the hi and low ranges. I believe you can go between the open and Loc positions (hi to hi loc or low to low loc) under 15 MPH or so. The open positions act like a differential; if it encounters resistance, that wheel will stop turning. Putting the case in loc locks the diff in the transfer case. (as I recall).

You might check out the 4x4 section as well. It might get a bit more traffic than the Blazer board, since your questions aren't so much Blazer specific, but more 4x4 oriented.