Another inexpensive option would be a Powertrax Lock-Right. I have one in my 92 W350. I got it for $255 new in the box off of ebay. I went to 4.10 gears when I installed mine, so I had to set them up, but pulling the carrier and putting on in your truck wouldn't really be a big deal.
I know you were concerned about street manners, and this is what I can report. Bottom line is they are cheap and they work. You get full lock up, and ratcheting action on the turns. They are LOUD. They clack and clunk like a Detroit. People will stare at your truck when you are turning into a parking space. They sometimes "catch" and chirp the inside tire on the turns, and therefore will wear more on your rear tires.
I've had them in 4 vehicles now over the years, and have put a combined 100,000 miles on them all in my trucks and in my Jeep - All with no failures.
The factory type one would definitely aid in traction and have better street manners, but a full on ratcheting locker will give you positive lock up and full traction to both rears all the time. It's kind of a give and take thing - just throwing out another option.
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92 W350 4x4 Cummins 5.9, 5 speed, 4.10 gears, Lock Right rear locker, 8' Flat bed.