ATX-Iroc

I just got this vibration in my car. one day just overnight my car started having this vibration at speeds above 50 and it makes this wierd sound right before it shifts into second it sounds like metal scraping. i got underneath and notice my rearend was wet with oil so i replaced it with a low milage 4th gen rear and the vibration and sound is still there. can u joints go bad overnight and cause a vibration.