First: I hope you did not pan very far.

Otherwise, you are going to have significant 3D movement happening to your sign. Motion can only do 2D tracking, which is fine for many things but might fail (i.e. look very unrealistic) for extensive camera movement. In this case, you will probably need a 3D match moving tool. If so, let us know and we'll point out some.
Second: If you have already mastered the tracking with simple text and are just annoyed by the background, you can always just create the complete sign (background and text) in some graphics programm and have that be tracked in Motion. Unless you want the text animated in some way...
Third: I am absolutely not a Motion expert, but I can remember there is a simple way to motion track several objects or layers to a single path:
You select the source footage and choose Behaviours->Motion Tracking->Analyze either from the Library tab or directly from the "Behaviours" button with the little cogwheel above your timeline. You can then add multiple tracking points with high contrast and analyze the motion. You will then have a track attached to your original footage.
Now create your sign background and your text. To both those layers, add the tracking behaviour by selecting each one and then choosing Behaviours->Motion Tracking->Match Move from the library tab. Now assign the source of this match moving to be the Analyze path you created earlier. Select the match move of an object and switch to the Behaviour tab in the Inspector. Do not create a new tracker and analyze the motion again, but rather just choose the already created tracking path as source. To do so, either choose your path from the small cogwheel next to the "Source" property of your Match Move, or just drag and drop said path to the small window next to source.
As I said, I don't have huge experience with Motion, but this definitely worked for me. Oh, I just found a very simple youtube tutorial on exactly this kind of motion tracking:
Simple Motion Tracking with Motion 5
Fourth and maybe most simplistic: Just put your Text and sign background in the same group and have the whole group move with the background. I haven't tried it, but I'm pretty sure this would also do the trick.
I hope all my menu entries were usable (my version of Motion is German, so I can't check...) and I could help some.