I need help with this :(

I dont have an example, but I will explain best I can. Basically I have some textual navigation that is vertical. When you mouse over a menu item, I want a box to slide (and ease) behind it and then when you mouse out it slides away…very simple, but I have yet to figure it out.

rollOut
[ text ]
rollOver
[||||text|||||]

Theres a simple way of trying to explain. The white space representing nothing behind it and then the ‘|’ character representing the box that slides (and eases) behind it. Thanks.

-Ronnie