How to trigger Jelly Bean Update
Read this from another site (was about updating to ICS) but I did the following and it worked for me.
Obviously have a Stock Xoom Wifi that is on ICS
1: Go into Settings --> Apps --> All
2: Scroll down to and select "Google Services Framework"
3: Click on Force Stop (let it stop) and then click on Clear Data
4: Hit your back arrow and go to About Tablet --> System Updates (should say it last checked in 1969) and hit Check for Updates
5: Let it download and install