That is likely an 8.2 carrier in there. I used an Eaton posi carrier and it was priced @ $398.00 from a performance shop in Knightstown Indiana in 2008.

 

I've had a Power Trax locker in my 10 bolt for over 15 yrs. and it works perfectly. Plus they don't require any resetting of lash or clearances. It installs in a few hours easily if you've got enough knowledge to change bearings on a 10 bolt. Just pull the axles, remove the spider gears and side gears, and install it in the place of the gears.
They run about $420 at Summit Racing.