What kind of trails are you planning on doing?

I have bud's skids with some extra gusseting and modifications over the years to strengthen them.

To your list:
I would say the transfer case skid is probably highest priority - there isn't one stock, just a flimsy piece of sheet metal bolted to the transfer case and it is right at the break over angle where you are going to get high centered on rocks all the time. Bud's skids also add a frame cross member here, which is critical.
Rear diff is pretty low priority. Protection for the pinion and e-locker motor are nice, but the rear axle can take a good bit of abuse. Eventually it sucks when you can't remove the drain plug as the ring around it becomes one with the drain plug.
KDSS and ABS sensors don't need anything, they are well protected.
Rear LCA mounts are high priority, they will get chewed up in no time. Also, rear LCAs will bend when you drop on them.

Honestly, unless you are planning on doing 5+ rated rock trails, the stock 4runner protection is pretty good and better than most give it credit for, much better than some aftermarket skids.