Who ever told you it is Dangerous, is correct, who ever told you to just add another Ground rod, don't listen to there advice again, as it is Dangerous ,you can not just add a Ground rod, if you did add a Ground Rod it has to be bonded to the main Ground rod this is the only safe way to add another Ground Rod, this could be more expensive to do than fixing your house wiring, as the Bonding copper wire has be sized for the job
You are most likely better to run a new line direct from the main Panel to your Garage