Rebuilding a Transfer case is no biggie even for a novice weekend mechanic. i'm guessing that you have a New process t.case. once you remove the t.case from the jeep, remove the front output yoke off and if the rear output shaft has that too, remove it as well. remove the speed sensor and switch. Find a place where you can set the t.case with the Rear of the T.case facing upwards. then start removing all the bolts that holds both halves of the t.case together. Gently separate the 2 halves to expose most of its components. check all the components and take note of their orientation so that you remember how they go back together. Your normal wear and tear parts will be the Chain, sleeve, synchro ring, fork pads, possibly the forks if the pads has been worn off for awhile, Gears , bearings and seals. Use your best judgement on what needs to be replaced, if you have doubts, take a good picture of it and post it here so we can give you a second opinion. as for the Loud rattle above your oil pan, If your certain that its not coming from the cylinder head area then i would inspect the Crankshaft main and rod bearings. if the noise is higher than the engine block, then it could be a colapsed lifter.depending on the severity of a Colapsed lifter , this could also be associated with a misfire, poor engine performance or a driveability issue in general. i may have left some other info. that needs to be checked, i'm sure our fellow members will advise you of that.