RE: How do I replace the gas tank float?
I have yet to hear of a float going bad in our blazers. More often than not, it is the fuel level sending unit which is part of the fuel pump assembly. In my experience, when a float goes bad, the gauge will always read empty because the float is full of fuel (in the case of the old brazed brass floats) and would sit on the bottom. I haven't ever had a blazer tank down, but I am fairly certain that the floats are now made of plastic which really can not go bad.
Now before you go pulling your tank, atleast try to probe the wires for the sending unit in the tank. Possibly clean the electrical connections and see how that goes. There is a circular indentation on the bottom of your tank. If you hit that with your fist (don't hurt yourself) with what you believe to be below a 3/4 tank, does that influence the fuel gauge reading?
If you have done all of this and it still does not work, then you will more than likely have to drop the tank and replace the fuel pump. I do not believe that the fuel level sending unit is independantly replaceable, which really sucks when that is the only thing wrong. Many on here have learned to fill up based on their trip odometer because their sending units have gone bad. Not a bad choice if you are strapped for cash.