Car barely runs won't rev over 4000 RPM poor power

3.8 motor almost sounds like exhaust manifold loose would not run at all. Replaced starter. Replaced plugs. Got running on 4 cylinders. Shop put in module under coils. Acted like exhaust plugged. And loud exhaust note. Replaced converter and muffler. Still has no power and sounds like exhaust or intake leak. Any suggestions please. Father of 4 needing his van to work.