1997 Suburban brake/cruise control question

I have a 1997 GM Suburban that does a couple of things that I think may be associated with each other.
Sometimes it won't come out of park once it's running (particularly on
a warm day) even though my foot is on the brake (but I usually CAN pull it out of park with the key in the ignition and into neutral to start it). Also, when going down the road with the cruise control set to 'on', the engine starts to occasionally buck - doesn't happen with the cruise control shut off. The guy at the GM parts counter wanted to sell me a switch assembly for about $15.00 but I couldn't figure out where this switch goes and didn't want to waste the money until I knew more. He indicated that it was one of those 'lay on your back from under the car (for lack of a hoist) changes and the switch is located under the steering column but in the engine compartment (rather than down where one might expect a brake light switch). Has anyone else had any experience such as this?
thanks, John
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What is this switch called?

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