Doing this:



Could lead to this:





 

How hard is it to replace side gears? Will I need to take everything apart and set lash and so on?

 

YES.
You don't have to remove the pinion gear, just flush very good to remove the fragments.  Everything else comes apart to replace the side gears.  Some have marked and removed bearing adjusters, bearing race, and cap together and replaced the still together group, and the the backlash should still be the same.  (I adjust mine every time it comes apart.)  Just keep the bearing adjuster, bearing cap, and bearing race, on the same side of diff as it came out of.

Acks http://www.acksfaq.com/ has a good write up.

I kept my side gears from my rear diff, but I always wondered how much difference it makes to mix side gears.  Like, taking side gears from a Sammy rear diff and installing them into that X90 for example.  My first thought is as long as the cross pins are the same length and thickness and the side gears have the same thickness and teeth, it shouldn't matter, but what about shims, what about differences in wear patterns?  Are these factors an issue or no big deal because the ring and pinion are carrying most of the load?

I haven't had a problem with mixing them up.  The pattern of the teeth doesn't seem to indicate a close tolerance.  The big problem is as hcgalvin has discovered when spinning your tires in air a lot of force is applied when they touch down.