From when I upgraded my truck to 4wd and talking to several dealers, those bushings are part of the frame. If you want new OEM ones, you need to buy a new frame.
I got mine from a junkyard with minimal damage and they appear to still be in good condition after a couple of years installed in my truck, I check them every now and then to see if there's any sign of them moving.
Unless there's something seriously wrong with them, just protect them from sandblasting and any chemicals you might be using.
2004 GMC Sierra 3500 C&C, added 4WD, 6.0L gas, Extended Cab, SLE, Transgo 4L80e-HD2 kit installed
Whee! Dually Club Member #1109