are you talking about the actual idler pulley or the belt tnesioner. if i remember correctly you have a M112 V6 engine, right? typically there is little to no shaking/virbration at all from the idler pulley so if you see it wobbling then change it...
to change the idler pulley simply take off the belt using a 17mm socket and rachet on the belt tensioner. then get a small pick or pocket flathead screwdriver and pry off the front plate on the pulley. then remove the allen bolt that holds the pulley on. install the new pulley and MAKE sure to install a new belt. i've changed idler pulleys before and still there was a vibration from the belt not be tracked straight. so i sell the two as a combo all the time now. the pulley is like $20 and the belt is like $27 from
http://www.autohausaz.com
easy DIY 10 minute job!
Jason