Change rear end gears, overdrive transmissions, and, auxiliary over drives.

Truck has the 2 speed rear and from what I read there is no way to change those gears.

Now if I got a single speed rear from a 1.5 or 2 ton truck are lower ring and pinion gears available?

Are there any overdrive transmissions that are a simple bolt on?

Are there an auxiliary overdrive units and are they simple to add on to the truck?