Yeah, that makes A LOT more sense. The rear disks alone are going to be more than $1k of that total, and then add whatever brake lines and modifications you need on top of that since Wilwood never includes the brake lines (shady bastards). Driveshaft is likely $300+ (what I paid for my last one with 1350 joints) and that doesn't include measuring time for labor. And yeah, an extra hour for the diff, an extra hour for the axles, several more hours for the brake conversion, and at a minimum $100+ per hour the "extra stuff" is probably adding a few grand to your total.
Plus the RMS is gonna take awhile. If that's a fully assembled car you're dropping off you've got to drop the fuel tank, peel back any fuel or brake lines in the way, clean the frame rails and prep for welding, weld the cross bar in, clean and prep the rear axle housing, locate and weld on the tabs. Doing it right means mocking it up fully, determining all the angles and lengths, then tacking it all together, taking it all apart again for all the welding then reassembly. It's going to be a lot of hours of labor if you're paying a good shop for the RMS install.
And then you're painting and undercoating, reinstalling the tank, hooking up all the lines again, etc, etc...
