550 uses a 135 series diff, with 4.88 gears. Unless you get a light duty 550, which uses a dana 80, in which case you may as well get an f350 and have it over with.
As it stands, swapping the 550 rear diff in, will not make your truck 550 capable, there are other differences in the frame setup as well. You might well find yourself a 2k 550 with a bad engine, and just swap that, you'll be money ahead.
Also, the 550 runs an 8x200 bolt pattern on the wheels and hubs, so unless you upgrade your front hubs, you'll be running two different bolt patterns on your wheels. The 550 uses 19.5s, not 16" wheels, and I don't think the 16's would clear the calipers on a 550, I could be wrong on that. I know a 16, or even a 17, won't clear the caliper guards on my 01 550. Just food for thought.