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Depends. There are a lot of heat treatable steels on the market. A heat treated 4140 or 4340 would work well. Fred's sells the steel spacers but I don't know if he makes what you need. I don't have is contact info either, maybe someone can post it.
Still, you may want to consider a gear change with your lift and new tires.
Why?
As an example, suppose you have 33 inch tires now, and 3.73 differential gears. You want 36 inch tires. Your new effective gear ratio becomes 3.419. To restore your gearing to the stock torque multiplication, you would need a 4.069 ratio. 4.10 would be as close as you can get.
Going to a 40 inch tire using the same example; Your effective ratio becomes 3.07 and you would need 4.52 to get back to the effective stock ratio.
There is a great set of calculators on this site that will allow you to plug in your specific numbers and help you determine if you do need to change gearing.