No need to change out your rear differential. It's fine. You only need to change the gearing inside of it if you change the gearing inside your front differential as well. If you don't, you'll have different ratios in your front and rear differentials and that would bind your drivelines when in 4WD. In stock U.S. configuration, both front and rear differentials are geared with 4.625 ratio ring & pinion gears.
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You only need to change your gearing in the differentials if you want to (you don't have to get new gears, but they are nice to have). Changing the gearing has no bearing on whether you have a locker or vice versa. You can get new gears. Or you can get a rear locker. Or you can get a front locker. Or you can do any combination of those. Or you can do none of it. It's up to you.
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These are the only requirements if you want to get an ARB locker for the front differential of your '87 Montero without doing a solid-axle-swap:

1) The only way to do it is by swapping out the front differential. The front differential you must use is one from a 1991 manual-transmission V6 Montero. I don't know if a 1992 or higher will work in a Gen I but perhaps someone else may know.

2) If you do the swap indicated above you will be required to use axles from a V6 Montero (I believe any Gen I V6 axles should do, but if you have a choice, get the axles from a 1991).

3) The ARB RD110 locker is the only front ARB locker available for Monteros. They list it as being only for the '91 and up.
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If you want a rear ARB locker for your '87, no problem. Just put in an ARB RD05 locker and a compressor to actuate it. Done.