I bought a complete Unirax rear end with a Quaife and 3.55's. It takes some machine work for the Quaife to be used, but it's very much worth it.
If you buy an assembly put together by a reliable source, the R&R is actually a DIY project that will take an afternoon.
This is the way I'd recommend doing it. If you can't DIY it, then all you have to worry about is the R&R labor and finding a 1/2 way competent shop to do it. The actual work in swapping it isn't at all complicated or difficult, but is a bit time consuming.
--FQB