Look for grease or something stuck to the inside of the wheel, throwing things off balance. Check the tightness of the lug nuts, but if you tried rotating tires I guess they are tight. Look for something that could throw the axles or drive shafts out of balance, or something wrapped around them. Otherwise, if they serviced the transaxle or diff, check oil level. Ask what brand and weight oil they used.
Current: 2011 Grand Cherokee Overland V8, 2009 Liberty Rocky Mt V6
Previous: 2000 Grand Cherokee Laredo I6, 1979 CJ7 I6 Quadratrac