I agree with BGH on NOT using ring gear spacers.
In addition to the differential that BGH suggested, I could/would also recommend the Detroit Locker (Eaton manufactures these as well). They are helical type gear locking differentials. They can be had new for around $470 before tax & shipping and are brutally strong.
Make no mistake I am not bad-mouthing the Eaton clutch type posis as they were the cats meow back in the day, and are still a good choice today. They are so close in price though when you buy them new I would just go for the locker. I've installed both the clutch type & Detroit Locker in several customers cars and never a complaint or problem out of either.