A friend of mine runs the shop up in Silver Spring. He did my 4.56 and powertrax no-slip install and no problems here.
Is the $4k price for the parts plus install? I'd say you should source the gears, install kit, and locker yourself from a forum vendor, then get the install done.
2004 Grand Cherokee Overland - "Couldn't keep it stock" DD
OME HD springs - Fox/BDS shocks - 255/70/17 Toyo AT2
1998 Cherokee Classic - SOLD - White - 5" lift - 35" tires - locked/geared/shafted/armored