Another bolt-in "upgrade" is the 3.55 Ring & Pinion conversion in the original 47-55 GM 1/2 ton enclosed drive-shaft rear end.
I went from Deve's 3.90 rear-end (which was standard in my 54) to the 3.55. I have never regretted this conversion, especially because I also replaced all bearings and seals. My Suburban is remarkably quiet at 60-65mph (I attribute this to good new bearings and a tight differential). The only reason I do not normally go faster is that my MPG reduces dramatically after 65mpg (wind resistance, I assume). I impress my friends (and win bets) by taking it up to 90mph for a few seconds a few times every year.
