As a back story I have an '02 4.0 TJ and a few weeks ago it started knocking. I had a couple people look at it and they can't figure out what's wrong. We took the valve cover off and they told me it wasn't the lifters. They believe it's much more serious and recommend an engine swap. So my questions are, Do yall have any idea of what could be wrong? and if I go with the engine swap where could I get a used one? I'm in dire need because my TJ is my daily driver. If it makes a difference I'm in southern OKC.