Yes, it is normal. Like everyone else said, I personally wouln't do it even at the 25 MPH limit Nissan set. At 25 it hits pretty hard. That's one of the nice things about manual locking hubs. Once you know the hubs are locked, you don't need to worry about your speed when you go into 4wd.

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Eric Konarske & Lara, my X
'01 Solar Yellow XE 3 Pk. 4x4