In the shop we would replace both. As a DIYer I think your best bet is to just do the passenger side mount. It is under more stress than the drivers side mount and fails sooner. It is also the easier of the two to replace and is certain to resolve your problem. It would be a good idea to check your trans mount while you are under the car. If it's bottomed out I'd do it at the same time. To check just pull down on the driveshaft behind the front flex disc. If it's got some spring to it, its still good. If it has no give or you can bottom it out then replace it. It's relatively easy to replace and I don't think it is as expensive as the motor mount.