Here's some aluminum spacers from an outfit in Riverside, CA for $109/pair:
http://www.trailsport4x4.com/Ford_Wheel ... -16x18.htm
My initial impression has been to go only with steel spacers, and stay away from aluminum spacers, because of the weight of these SMB's.....but after a bit of consideration, I'm thinking there might be exceptions.
As in: for a 'lightweight' 2WD RB that -doesn't- carry 10,000 pounds of gear (or anything over 5,000 pounds on the rear axle), and more likely hovers around a total vehicle weight of somewhere between 7,000 and 8,000 pounds......I now am starting to get the sense that a well-made set of aluminum spacers *MIGHT* be ok. (Bearing in mind that they would need a slight bit more attention to re-torque the lugs early on, as you have to do with almost all new aluminum wheels (and some steel ones....))
I've talked to the guys at Trail Sport, and they are very positive about the strength of their product, and its ability to more than hold up to the weight of an SMB. They claim to have done a ton of product testing. Their spacers are 6061 Aluminum, Grade 8 bolts. In a 2-inch spacer they sound pretty robust, he was comparing their actual "in-use" strength to be nearly 90 percent the strength of the steel spacers, but with a lot less weight. He claimed they've had guys with 1000-horsepower dually rigs doing severe testing with them and zero failures.
I'd be really interested to hear anyone else's more-experienced opinion here on these.