Alterntively, you can also use our extension Pins & Boxes - in particular if the shape needs to be behind multiple objects:
1. create 4 pins at the corners of the text
2. create a trapezoid path and use the "Nulls From Paths.jsx" script that is included in Ae to create 4 null layers at the 4 vertices of the path (choose the option that the path follows the nulls, such that moving the nulls moves the path vertices).
3. parent the 4 nulls to the 4 pins - now the path resizes with the text.
If you need rounded corners, you can use the Connect Multiple Layers iExpression instead of the Nulls From Path. It can also create paths whose corners are connected to null layers, but with an additional roundedness parameter.
The basics can be solved with a couple of expressions if you use a shape layer rectangle and use Skew to generate the trapezoid.
I've uploaded a sample comp with the expressions. If I get time later today I'll add the paragraph justification code. You can then same the shape layer as an animation preset to create multiple copies with a single click. The shape layer always needs to be just below the text.
You can add sourceRectAtTime().left to the mix to compensate for Left, Right, and Center justified text.