I use ECGS. They send you assembled thirds in buckets, you install and send your old ones back to get the core charge removed. No down time, back on the road the same day. I think it's around $550 per third total for a regear (more for a locker), and they come with a 5 year warranty. Of course, it's a lot cheaper to do them yourself if you can.
RING AND PINIONS
Zuk is probably the most knowledgable gear installer on the planet, and he documents every build. You send your thirds to him, he regears and sends them back. This is the best option if you have time and want the best setup possible.
Toyota Gear Install Harrop