So I come from a place that it did not snow so last winter I bought my f150 with 4x4 since my old truck didn't have it.
Questions are:
What speeds are safe for 4x4?
Can you change to/from while driving and if so what speeds are safe?
What is the difference between hi and low? (When I turn to low the light doesn't turn in only for hi)

 

It will not go into low range unless you're in park or neutral at a stop or crawling speed. Low range is for low speed high torque applications like pulling a boat out of the water.
I don't think travelling speed is important but shifting should not be done under throttle. If i shift on the go i try and coast(any speed), shift, wait for for high light plus another three seconds ish to fully complete the shift, then just drive normal.
Good luck and enjoy your new truck.

 

Read the manual, it will tell you more than you need to know.

 

Yea I have done it while coasting and no issues just wanted to make sure that's not a no-no