ARB does make a locker for the rear. For some year models they already have a locker for the front, too, but Dodge changed the set-up a couple of years ago, and they are just now getting around to making one for the front for those newest models. E-mail them at info@4x44u.com to get a phone number for their U. S. Distributor and answers to any other questions. I'm not sure which set-up the 1999 had, but ARB will help you - they are very customer-oriented.
Lockers on the front end are generally not advised for safety reasons. The ARB is an exception to this rule because it is only engaged when you push the button.
For the rear, most of the lockers can be fitted to your axle, regardless of whether you started with a factory limited slip or open diff. But if you already have limited slip, find someplace to test it to see if you are happy or not before spending more money. Yes, some people don't like the factory LS, but, many people do like it. There are still some threads on this board that discuss this at length; try searching on "limited slip".
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2001 ETH, QC, LWB, 4WD, 3. 54, SPA Pyro & Boost, K&N RE-880 w/foam pre-cleaner, ARE Z-series Cab-high shell, Rear ARB locker, and all kinds of synthetic fluids...