Just in case anyone is interested....the front differential from a 700 shares many common parts with the differential from the 500. Looks like an additional case or two and some locking shafts, bearings, and small parts to turn the 500 diff into a locker. Or a front final drive assembly from a 700. Should be less than $1000 if you truly have to HAVE a locking front differential.
I am not saying everything with fit perfectly either but it looks like it will (same part numbers in common). The biggest hurdle would be how to engage the locking differential or just leave it locked.