I have a 91 Cherokee inline 6 with a decent oil leak, I know that some is coming from the oil pan gasket but I think it’s coming from somewhere else but I can’t tell from all the sludge built up. How can I clean all of the sludge off easy and is there common places that oil will leak from?

Start at the top and work your way down, as most of the leaks end up dripping from the back of the oil pan. I like to get the engine warm, spray the engine down with cheap engine degreaser and let it sit for a hour before hosing it out. Repeat as necessary for heavy build up. Be careful around the throttle body, as the TPS sensor is know to go bad if it gets pressure washed or cleaner inside it. A pressure washer can be helpful, just keep in mind that it's can easily inject water into connectors and places you don't want it.

The most common places for a leak are:
Valve Cover - Check first. Usually the back of the engine block will be oily. Often mistaken for a rear main seal leak
Oil FIlter Adapter O-Rings - Pretty common. Usually gets the side of the engine and the starter get covered
Rear main seal - Pull the inspection cover off the bellhousing and look. Confirm it's not from above first
Front Crank seal - Often the HB needs replaced at the same time.
Head gasket - Not that common, but I've had/seen some that slowly weeped oil along the passenger side. Not really a problem, just a cosmetic issue.