Sounds like you may have a possible misfire, replace the plugs and wires if necessary. You may need to have it hooked to a diagnostic scan tool and see if you can isolate a malfunctioning coil as well. Another possibility may be your motor mounts, over time the rubber mounts can soften and cause a vibration at low speeds.
〉 Answered on Aug 14th, 2008 by Amanda J. Pierce, MBA "AJ", Certified Mechanic, Project Coordinator; Primavera Scheduler at VIGOR Alaska Industrial Shipyard & BizzM3ch Solutions
You may need to have your tires balanced, or it could be a bad tire. There are many things it could be, you might want to have a qualified technician inspect it.
〉 Answered on Aug 14th, 2008 by Donna Burmeister, Service Manager at Classic Nissan