You use a dial micrometer and good grief there are 10,000 references out there on the internet. The 8.8 has been covered on numerous Ford and Jeep sites. I don't think anyone wants to take the time to try to explain it all to you in a post.
Summary: you mount the dial micrometer to the back of the diff with a magnetic base, set up it's tip tangent to the ring gear just touching a tooth, and with the pinion immobilized you rock the ring gear back and forth and see how much it moves. Based on that you reshim the ring gear until the lash is correct. When you are done, it must be TIGHT. You should barely be able to get the shims into place.
Here's a topic with some of my experiences rebuilding an 8.8: http://www.ranger-forums.com/forum2/...ead.php?t=6064