form element highlight

I have a webpage with a <form> consisting of a large table grid of dropdown
lists used to make changes in a database. Some of these dropdown lists have
the same value. I'm trying to make it so that when a user mouses over one
of the dropdown lists, that dropdown list, and all other dropdown lists
having that same value are highlighted with a yellow background color. Then
on a mouseout the yellow highlights go away.

The goal is to it make it easy for a user to graphically see where all the
same valued dropdown lists are in the grid.

Ultimately when a user makes a new selection in a dropdown list and changes
its value, they are then able to mouseover that dropdown again and see same
valued dropdown lists based on the newly selected value.

I hope that makes sense.

Thank you in advance for any assistance you might be able to provide.

Dave