Car,
Heres the way GM taught us to do this procedure.
BEFORE you remove the pinion nut,mark the nut and pinion shaft with some touchup paint.The kind auto parts sells with the brush attached,I prefer white.
Then use an impact wrench to remove the nut.
After you remove the old seal and re-install the new one,you just run the nut back to the marked position you made with the paint.
You will know this by feel,once the nut stops don't run it in any farther,then check and make sure you are where the marks line up.
Now run the nut down another 1/4 to 1/2 turn,this will ony take afew quick berps of a good impact gun!
And will restore the factory pre-load to the crush sleeve,I always put a drop or two of locktight red before I re-install the pinion nut...
Just remember,do not over do the tightening process,and you should be fine.
As a flat rate tech,I have done this hundreads of times with no problems of come backs from rearend noise.
Hope this helps,KS-out.
http://photo.starblvd.net/zanko20
[This message has been edited by KS circutguy (edited 05-31-2001).]