Different rear ends are an option from GM Fleet. Gov't agencies order each new car and have to wait for it to be built, it's not like they buy them off of the lot (most of the time, at least). 3.08 vs 3.73 has to do with the transmission. Older trucks have 4spd, and got 3.73 standard. Newer trucks have 6spd and get the 3.08s. Extra gears = taller final drive ratio. There are two versions of the police Tahoe, too. The PPV and the SSV. The PPV is 2wd, sits lower, and is pursuit rated. The SSV is 4wd, normal ride height, and is not pursuit rated.
As for weight reduction, cam, engine mods.....no. An emphatic no. Everything is bone stock. I laugh at the idea of weight reduction, actually. It's quite the opposite. If you had any idea how much extra shit I have to haul around in my work expedition, you'd laugh too. The TruckVault in the back weighs several hundred pounds by itself and would negate any other "weight reduction" stuff on it's own...and that's not even scratching the surface.