What would make a 1995 Chevy 2500 gas gauge peg way past full and stay there?

It could quite possibly be the sending unit, that is located in the fuel tank. The electronic "board" gets crudded up and you must drop the fuel tank and pull out the sending unit which is part of the fuel pump. It's best to have as little fuel as possible in tank when doing this but It's not that difficult if you have a little mechanical ability. Two straps hold tank to frame. Drop tank, pull unit from top of tank, clean or replace.