Hi RizBajwa,
Welcome to the club.
For an old Pathfinder, you're best bet is just do some googling for an economical rear locker. For the money, I went a Powertrax No-Slip unit in my old Cherokee XJ. Fantastic little unit, cheap, and because it's an auto-locker, it's essentially 'always ready'. It only kicks in when one wheel starts to spin with no need to engage on my part. My theory was... when I need locking, it's already too late to engage it. And it directly swaps with the spider gears, so easy to pull back out and return to oem if needed.
Any diff locks that require engaging are going to cost money and complications, as they require either an electrical, hydraulic or pneumatic system to support. It just depends on budget, as you say... ARB can be pricey.
For workshops, I've used Icon 4x4 in Dubai. They work on anything (no joke), are dedicated to the offroading market, and their team is solid.