2005 Ls Chevrolet Avalanche two wheel drive
Unless you have mad skills, you won't be able to fix it. Most likely, either the reverse band has gone out or the servo/plunger which operates it is no longer doing its thing. Either way, you'll need to take it to a shop to have it fixed. Reverse band is pretty much a rebuild. Getting the plunger fixed is hit or miss, but can be done by itself.