Just to be clear, the "Spartan Helical" is not a locker, it's a mechanical type (as opposed to a clutch type) limited slip differential or LSD, often referred to as a Torsen. I also have one in my front diff and it works in both the Dana 30 and the Dana 30a. It was $259 off Amazon so very budget friendly. The Eaton Detroit Truetrac 912A585 is a similar design - some trust it more than the Spartan since it's been around longer but it costs more. When I was shopping around it seemed that putting a real locker in the 30a was not recommended since they're so fragile. I've had no issues with my Spartan other than that it doesn't perform like a locker when you need it to - when one front wheel has traction and the other has zero traction the one with zero traction spins uselessly and the one with traction doesn't do a thing. You're supposed to be able to hit the brakes to load up the spinning wheel and then the wheel with traction starts moving but I'm not sure how well that works. It seems to work better than an open diff but not as good as a locker.
I also have a Torsen type LSD in my rear diff but it's a Powertrax Grip Pro GT308229 (for the Chrysler 8.25). I paid $495, I think off ebay. It hasn't given me any issues either other than the same problem described above. I did the rear first and I got stuck on the trail in a couple of situations - applying the brakes didn't work very well, maybe because my rear (drum) brakes suck and I probably just locked up the front wheels so they ended up doing nothing. Maybe applying the parking brake would have been the key but don't think I thought of that at the time. Anyway, I wish I'd done a real locker in the rear which would have given me a more trail-capable Liberty. I may go that direction in the next year or 2 but not sure what type of locker I'd go for.
Both my LSDs will accept 4.10 gears but I'm still on 3.73s.
Make sure you identify your rear diff before buying anything - is it a Chrysler 8.25 or a Dana 35?
As duderz7 says, just google "jeep liberty locker" or "jeep liberty locking differential" or "dana 30 locker" or "chrysler 8.25 locker" or "dana 35 locker" (if it turns out that's what you have in the rear). There seem to be multiple styles of locker out there - there are some good videos describing the different types that I watched a few years back I'm sure you could find.
Also, the installation work is not trivial, especially for the front axle (it has to come out of the vehicle) - if you're going to regear at some point I'd do all the work at once. Installing lockers / LSDs now and then coming back later to regear would be a LOT of labor.
