Based on the limited experience I've had I'm not sure there's a precise answer to your question. One driver's 4-lo/diff-lock experience is a 4-hi unlock cakewalk for another.
Read and experiment is just about all you can do. Maybe other more experienced drivers can provide better guidance.
A few reading ideas that may give you a starting point ...
Hummer X Club tech doc.
Operating the H2 Transfer Case .
Four-Wheelers Bible by Jim Allen.