I did the same thing on my rear end (put a Truetrac LSD in my Dana rear end). We checked backlash before loosening anything -- it was within factory spec. Then we set the shims up on the new diff exactly like they were on the open diff we removed. We lucked out -- it went in, and the backlash was exactly as it was with the previous diff.
But -- you need to measure, and be prepared to re-shim because it may be necessary.