Theoretically in 4WD Hi you can drive at any speed you want (provided you are not turning), however most of the time conditions will dictate that you not drive very fast, otherwise you wouldn’t be using 4WD. For low range due to gear reduction via the transfer case you’re going to top out around 30-40 MPH for most vehicles (redline limited). But driving that fast in low range may cause damage to drivetrain.