Use of back of and in back of to mean "behind" is widespread, but opinions differ as to their respectability. Each should be considered standard, but both are wordy and can safely be omitted in favor of behind: "The wheelbarrow is behind (back of or in back of) the barn." No one questions the reputability of in front of, although before is shorter and normally will serve as well.
Dictionary of problem words and expressions. Harry Shaw. 1975.