I have a 8.8" with "posi-trac". You need to run the Ford friction modifier along with the diff
oil. There are clutches in the track-loc that need the modifier to get lubricated. After adding the modifier, turn the car in tight left and right hand circles, (not doing donuts), about 8-10 turns in each direction.
Here's a couple sites that have it.
http://www.mustangmagicparts.com/m19...-mod19546.html
http://www.stangmods.com/M-19546-A1-...er-p/00364.htm
I used redline shockproof gear
oil in mine so I used their friction modifier from Summit.
http://www.summitracing.com/parts/RED-80301/
Hope this helps.
Dan