You can have the hubs locked full time all the time, no probs.
The front axel, just like the rear has a differentialential.
The role of the diff is to allow the wheels to rotate at different speeds.
The frontera, not having a centre differential between the front and real axel
means that 4WD should only be used when the ground is louse or slippery .
This allows the wheels to slip, allowing for the different speeds of front and rear wheel rotation, (cornering and tread depth etc)