What is causing my car to make a clicking/knocking noise?

hi, i have a 1996 pontiac firebird formula and recently i have been noticing a clicking/knocking noise coming from the back end somewhere i hear the noise from inside the car after it warms up and has been driven for about 5 to 10 minutes it only happens when i accelerate and speeds up or slows down depending on my acceleration what i mean is if i accelerate slow it clicks/knocks slowly but as i accelerate faster the clicking/knocking noise speeds up faster i usually hear the clicking noise between speeds of 1 to about 25 to 30 mph i think i usually dont hear much after going faster then that. I looked under the car the other night with a flashlight and i noticed something what looks to be oil on the driveshaft where it connects to the differential so im thinking i might have a small leak where the differential connects to the driveshaft that could be causing the differential to be low on oil causing the gears in the differential to click around im taking the car to a gas station tomorrow to have them check the oil in the differential i was also thinking maybe something from the back wheels could be causing this too. and when i back up sometimes it sounds like something is catching the tire when is spins causing the clicking noise but when it makes the noise backing up it makes more of a grinding type noise but not like a metal grinding noise more like something plastic is grinding up against the tire or something. so if anybody could give me some thoughts on some possibilities you think it could be that would be greatly appreciated! Thank you so much for taking the time to read this and i look forward to hearing some possibilities.