Could be a bad tire, they are the about the most vulnerable thing back there. In all my years with Jags, this is the most common cause of vibration, tramlining , pulling and so on. So many parts have been replaced when the tires were the culprits! A tire can look perfect and be the crummy part from hell.

 

Describe the shaking, when it happens, intensity, where at, what exactly you have done so farand so on. You have given too little information to get dependable help