I have a 2' cast iron drain pipe from my kitchen running under a patio to the grease trap. Now it appears that it has a leak somewhere either under the patio or just past where it goes out of the foundation. My question is what is the best way to replace the pipe, short of tearing up the patio?

what is the best way to replace the pipe, short of tearing up the patio?
Dig down to the cast iron and inspect the pipe. You can tunnel under the patio deck for a ways. Try to locate the leak and tell me if it's from an bad lead joint or a cracked pipe, Back to you, Tom