oil pump replacement

I have a 2000 Jeep cherokee and am having a problem figuring out how to get the oil pan off. I believe my oil pump isnt working correctly and i am running low oil pressure. at idle, the oil pressure drops off completely at times once the engine is warmed up. if its the main bearings to blame...

I have an 04 Grand Cherokee Limited V8. For a couple weeks now when I go to start her up the check guages light is on and the oil guage is reading at low and the engine sounds a little "rough"; after about 30 seconds or so it goes up to a little above the middle line and the engine sounds fine...