unsteady on back legs

i have a 11 mothes old cross yorkye jackrussel she is verry unstable on here back leds she is full of energy but it is like she is 2 dogs the front legs go in one direction and the back are still trying to find out were the front legs went it is like she is drunk but just here back legs she is a nervous dog but she is very loving but jumps at loud noises out side if you can help me find out what is wrong with here it would be most appreaciated thank you