Q: Noise in the left side rear when drive on a no flat street

Hi Carol...the best results are obtained, and costs for everybody are kept to a minimum, if you ONLY replace parts that are confirmed to be defective. If you have not yet gotten a conclusive, authoritative diagnosis from a certified mechanic that the shocks are bad, talk of replacement is premature. In particular, you could have bad shocks but the noise you are reporting originates elsewhere in the suspension and so replacing the shocks will achieve nothing if your objective is to get the noise to go away. The bottom line is your best bet is to schedule an inspection of the shocks, during which inspection the Mechanic can let you know both if the shocks are bad and if their failure accounts for the noise that you are hearing. If the noise is not coming from the shocks, the mechanic will tell you where the noise is coming from. As far as whether shocks are replaced in pairs, the answer is yes they are. Anytime one shock on one side has failed, the other (on the other side) is almost undoubtedly substantially worn out if not totally failed. What I strongly suggest is you just set up a shock absorber inspection/replacement and during that visit if the shocks are bad, they will be replaced. If they are not bad, and yet you are hearing a noise, the origin of that noise will be pinpointed for you. As a general rule if your vehicle has over 100,000 miles, the shocks are definitely "worn" and it is a good idea to just replace them. Whether the shocks have "totally" failed or not would not be clear unless the shocks were inspected. Shocks will typically degrade somewhat gradually over the years although there outright failures do occur due to large sudden leaks and so forth. If you schedule the inspection, the mechanic will thoroughly address all of your concerns and get the needed repairs done. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.