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How high is the speedo reading? There is a mechanical gear in the transmission that turns a sensor, changing the rotational action into an electrical pulse, which is read and interpreted by the cluster to give you a speedo reading. I can't think of anything they could do to make it read HIGH, only low. But if you have a fault set for this sensor (or pretty much any engine fault) the cruise control will shut off.
I would bring it past the shop again, explain to them that this has happened since they did all the work, and could they scan it quick and see if a sensor or something got unplugged? Don't act too irritated, just act like it is no big deal to have them "do a quick check of their work". If you go in mad they may get defensive, and if you go in too meek they may try to take advantage of you, claim that they will have to charge you for diagnostic, and try to claim it is not related to their work. If you act like it is just a fast check to plug a sensor back in, you may even be able to hang out while they check it out, so you can make sure they are not trying to cover up a mistake or anything. You know what I'm saying? If it's a dealership they are usually pretty good at covering things like this, to retain customer satisfaction, but often indie shops are hit or miss on customer ratings.