I had an Aussie Locker up front for 3 years and it performed great off road! It was totally transparent on road while in 2 wheel drive mode. Once shifted into 4x4, the Jeep was almost un-steerable on pavement.

You might consider a Detroit (Eaton) Tru Track. Its not quite a locker up front but its pretty darn good. And you can still drive it in 4x4 on the road, which is nice.