Soil Sifter with 30W Motor, Metal Sifter for Rocks with Switch & Noise-Reducing Rubber Pads, Automatic 39" x 19.5" Large Compost Screen Sieve for Sand-Idea for Most Wheelbarrow Carts (Not Included)

A rototiller just will mix the soil up, putting the gravel in your soil. What you want to do is get one of these: soil sifter and painstakingly shovel each bit of gravel up, run it through the sifter, put the dirt back, and haul the gravel off somewhere else then you either rototill to loosen up what is left or you plant daikon radishes in the entire backyard and let them till it for you. Alternatively you pay a company to come in with equipment, scrape the top 3-4" of gravel off, haul it off, and deposit 2-4" of topsoil in its place. Which won't be cheap, but it also won't be the most expensive landscape project as far as such things go.