U-JOINT! put it in neutral and push and pull on the driveshaft at the ujoint, if its bad you should feel and see the slack. If not fixed soon It can come apart while driving it, ive had that happen. Autozone (of all places,lol) has a ujoint called bruteforce and it is the best ujoint I have found other than a spicer joint. Last time I got one it was about $15.