I'm trying to locate my oil leak?

My van has been leaking oil for a while. Started out just a small leak. Now it is leaking pretty bad. As far as I can tell it is leaking from somewhere in the back. It only leaks while I am driving. Oil gets on the exhaust manifold and burns off and is pretty much all over the under side of van.I just changed the oil sending unit, thinking that may have been the cause, but it is still leaking. I had a mechanicly enclined friend look underneath while the van was running and he said he could see oil leaking from what he called an oil return line toward the driver side wheel well. I can't find any info on an oil return line and was wondering if you may be able to tell me if such a thing exist. Or what be other possible places it might be. I am a women and the mechanics around here just want to play the guessing game on my $. Please help