Drive shaft and rear end

[9Cobra9]

i have a 99 cobra and i want to get a aluminum drive shaft for it. i want a frpp i couldnt find one specifically for my ride at 50 resto. the part called for i think96-04mustang and 96-98 cobra would this work? and another thingi have a slight vibration i the rear and i wander if my pinion bearing is goin bad. it does not leak while sitting and it hardly ever leaks while running but i have seen a small run spot on my muffler from what it looks like at the seal. I have wiped the run off and it has not come back but u can tell it has alittle grease around the seal. could it also be that i have a driveshaft out of balance, i am just curious it dont vibrate while idling down it just does it when u get on it or when its in a higher gear and u put a load on it goin up a hill.