If you can rotate the prop shaft 1/6th of a turn (near the rear diff) the backlash must be a mile out of spec? or the rear diff`s worn out. I can turn mine 1/8th and that`s enough for a clunk from the back.
The limit`s 5mm on the diff casing (see link), that`s probably 1/16th rotation or less!
Backlash Testing
If you`ve had it set up at a garage I`d be taking it back personally.