I don't know if you're talking about axial runout or what so I'll assume the worst. Please make sure you are dealing with perfectly clean surfaces on the back side of your ring gear and where it mounts. A bit of anything can cause this to happen and I exclude nothing...one grain of sand will do it, a nick or loose shaving will do it...anything at all (even grease from your fingers containing dirt, etc). Anything that might lift one side of the ring gear higher at all. Since we have no idea if the numbers you gave us are starting at .010" and rotating clockwise or counterclockwise and ending up at .008" that's a question mark for us but please remember a sheet of common copy paper is .004" thick. Also...and I mean no disrespect at all...are you using a very good dial gauge? I'll assume the answer is yes, and I'll assume .008" is the low point and a bit of schmutz is diametrically opposed to that (somewhere around where the .015" reading is). That's where I'd start my witch hunt. Good luck!

As a side comment, these numbers you gave us are not really all that bad. If your target is .010" and everything is perfectly clean, I'm not sure I'd make too much of a problem out of it. My rear end came from a 1969 Chevy pickup and if I had backlash of .040" I'd be dancing around the block singing but it works okay.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end