Used ASD/LSD diffs will be useless, as they will also need a rebuild. Trust me... I bought two of them. Both were 99% open, 1% locked. The one in my car is the same way. I'm working on rebuilding it myself right now. The parts will cost $300-$400 total, plus some special tools (which can be fabricated). A good rear-end shop should be able to do this for more like $1000, not $1500-$2000... that sounds really high. ASD diffs have 35% lock (when new!) without hydraulic pressure, and near 100% lock with hydraulic pressure.
Photos of my current project are here:
http://www.w124performance.com/images/W124_diffs/185mm/
I'm hoping to have it back on the road by the end of May.