What is the difference between a 'live' NLG axle and a 'fixed' NLG axle?
A live axle is when the two wheels rotate together. Its function is to reduce shimmying.

The Lockheed JetStar and the Space Shuttle Orbiter have live NLG axles.



References:


Source: Aircraft Landing Gear Design: Principles and Practices


  The nose wheel is a split, forged aluminum alloy design but its bearings are located on a rotating or "live" axle rather than in the hub such as in the main wheel design. Both nose wheels are splined to the "live" nose axle providing a corotating feature which improves stability or reduces the tendency to shimmy.


Source: Orbiter wheel and tire certification, NASA


  Tire friction can also be used to increase the damping in the castor mode. If the gear has two wheels, connecting them by a live axle can have this effect. In the case of a single wheel, it is possible to obtain ‘anti-shimmy’ tires incorporating two contact areas.


Source: Landing Gear (Aircraft), sciencedirect.com