Best thing to do is buy a complete diff / bulk assy off ebay and then reshim it. The k series diffs with the bolt on ring gear are not the best, but if you shim them well they will last. As neil said get the pinion shimmed so that it meshes with the center of the ring gear, then shim the diff to fit snuggly against the pinion. If it feels a bit notchy that will be fine as the gears will wear together when you run it.
Link to part I mentioned :
http://cgi.ebay.com/NEW-OFNA-ULTRA-L...3%3A1|294%3A30