IF you only go one locker it should be in the rear, or first one that gets a locker IMO. If the rear is open (or l/s) I couldnt expect front to pull truck if back isnt pushing. Though it would help, if the rear is spinning a tire thats putting a lot of load on front drivertrain (weaker of the 2).
An ARB is an awesome front unit-being able to take it in & out as needed (without load). But normally you dont 'drive' with it engaged full time or at least I dont. As to turning, rear locker pushes me straighter than the front, but front locker does add to effort of turning steering wheel.
Your not 'wheeling' this unit so probably wont be mechanical issue adding to front. Hopefully youve at least a rear l/s. As to leaking older units were more prone but clean air and clean fluid you shouldn't have a problem. But incase for that unlikely event Do reroute the axle breather tube higher and where can easily be seen and away from are you dont want oil. If it should start leaking, again unlikely but its simply o rings allows air to enter diff housing pressuring, any fluid in tube will be pumped out. Usually minimal spitting burping fluid but I have seen one complete failure and pumped diff dry. But that was bad install-