There are many different valves options in the market, depending on applications.
we installed one on the Hino Expedite truck about 2 years ago, & my new trailer was spec't with one.
it can be as cheap as a 22$ line valve, & up to 6,500$ for a drive lift axle option.
you see, the problem with older tandem drive axles, is the differential locker was not build for that option.
in the new Peterbelt tractors, the ones with the disc brakes, its just a matter of tweaking the truck computer & installing the EOM dash switch.
my OTR carrier insists on installing those on long contractors trucks * at the time of orientation. makes serving it's costumers better then our competition, suddenly everyone's impressed with our drivers backing skills.
(*) the truck have a longer wheel base, not a tall driver...