I ordered a Ford F550 4X4 in December 2002, and took delivery in January of 03. It has the infamous 6.0 diesel which never really gave me trouble until last spring. After months of fix this and then fix that, and at a price north of $9,000, I finally got it on the road, hopefully for good, this past week. But, I will say, probably 80% of the problems I had were electrical in nature...up to and including the battery terminal & cables giving grief. It has 160,000 miles, all under load on it, and it runs just as strong now as it did when new, and doesn't use a drop of oil. I use Royal Purple synthetic 15/40 in it. The Torq-shift automatic works like a champ. It's never been touched only to have it serviced with the BG products every 35,000 or so. Average mileage, under load, is around 11-12 mpg. I did drive it to Omaha once with the bed bareback to pick up a bunch of flooring, and empty it beat 16, barely. With the 4:88 rear end, 60 mph is about fast enough, but you can wind it a lot tighter if you want.
When I say under load, I mean with close to 10,000# on the bed in liquid feed, tanks, skid, and pump & motor. Going through all kinds of pastures, stalk fields, bad roads, etc. in all kinds of conditions. Almost abuse, but not quite. If the engine & tranny stand up, I don't think you could EVER wear one out. They should work plumb fine under a bale bed and last forever. I would not hesitate to get another F550 if I needed one. I've stated here before, though, that I would probably go with a gas engine next time, due to the fact that gas is over $1.20/gallon cheaper than diesel right now, and there is probably no difference in mileage with the way I use one. I will say that what warranty work was needed, my Ford dealer stood behind it 110%....no issues there. My truck is an XL, rubber floor, vinyl bench seat, AM/FM radio, and crank windows.... I wanted it that way...less crap to give me grief. And remember, FORD NEVER TOOK ANY BAILOUT MONEY!
All I know about a 5500 is it's a Doodge. I couldn't bring myself to drop the hammer on a Doodge no matter what, but that's just me. I think Soapweed has a relative or a neighbor that has a 550 with a Hydra Bed. I'm also not a fan of the 19.5 tires, cause they ain't cheap & you need six of them, but whatchoogonnado? You won't be disappointed if you go with a 550. I sure haven't.