Phil, definitely try it out of the truck. I suspect (could be wrong) that if it has 4.272 gears you'll need the smaller actuator. I had a thread going a while back with all the part numbers in it.
If you can't get a new part, you may find it's leaking around the metal joint that holds the diaphragm in place. If so I'd RTV it (permatex gray) to give it a longer life.
'97 Montero 'LSR' - 4.6 gears & factory rear locker, 33" Procomp muds on 15x8 steelies, 50mm coil spacers, T-bar crank, Airlift, sans rear sway, 50mm rear frame & fuel tank lift, Aisins, ARB front bumper + 10k lb winch, 50mm DIY body lift, rock sliders, cut rear quarter panels...