Leaky diff should be as simple as replace the seal and fill it back up, no need for the IRS to come out (well it SHOULD at some point to replace the crap rubber bushings with Delrin). Leaky trans might be the trans, you might have a rear main leaking, clutch change will vary from place to place. A good mechanic can do a clutch pretty quick all things considered. For the clutch itself I'd just get the $160 King Cobra kit and have them install that.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.