Does it need to pass emissions / inspection in your State? That's usually the biggest consideration to doing engine swaps - being sure that base is covered.
I haven't done the 3.4 swap, but I have and am in the process of another 4.3 Vortec swap. The 4.3 is similar in execution, but the 3.4 is much easier.
I'm sure you already know that the 3.4 is a direct bolt in for the 2.8, you can use your old exhaust manifolds and accessories, it's the intake and fuel system you need to figure out.
If you're savvy enough to get a GM fuel injection system working in the XJ, that's the route I'd recommend, but if you want it carb'd, then you'll need to get an electric fuel pump, intake manifold and decide between 2bbl or 4bbl carb.
Going the carb route is the easiest and possibly least expensive.
Tell us if you're going F.I. or carb, then maybe we can steer you in the right direction. I recall a thread or two that had the intake manifold part numbers from Edelbrock, and some carb info somewhere......