We have video instructions that are pretty clear, and where always here to help with your questions. With that in mind any internal differential maintenance is a task that requires some technical skill.
Yes, if you are not experiencing differential wheel spin it is ok to engage the HALO locker at any speed.
When the locker is off/unlocked your vehicle will act normal. When in lock the differential has a 100% positive no slip lock. This will cause your steering to feel stiff and reduce your vehicles turn radius. For these reasons it is recommended to turn the locker on only when needed.
A locker is basically a slang/abreviated term for a differential locker and sometimes referred to as a pin locker depending on design characteristics. The lockers function is to lock both drive wheels of an axle together. Without the drive wheels locked together the differential may not be directing power/torque to the wheel you need it …