Don't get me wrong but I don't recommend just throwing random parts at it. A decent tech will do some testing and basic circuit trouble-shooting before unloading the parts cannon on something. Do a little research on what the parts actually do and how they interact. For the brake pedal switch you replaced, doesn't that control if the brake lights come on an off? If it wasn't working wouldn't you have issues with the brake lights not coming on, or not coming off? Also not sure how the brake pedal switch would cause the trans +/- buttons to not work.
I know a lot of people don't want to take their trucks in to a shop for various reasons, usually either trust factors or they don't want to pay the money. But if you don't have the knowledge, tools, or willingness to do some research on how the systems work it's often hard to figure out the issue. It's kind of ironic on the money because I see people that don't want to spend the $300 diagnostic fee to figure out exactly what is wrong, but will spend $900 buying random parts to throw on the truck and hope they get lucky. The trouble is that the exact issue and symptoms going on with a truck often don't get communicated well enough to determine if it's actually the same problem or not, and on top of that the same issue or symptom can be caused by different things. Like in this case, somebody makes a post that their cruise control is not working and another person is like "yeah, I had issues with my cruise control and replaced part X".....the trouble is there are a lot of different things that can cause a cruise control problem and you can't even be sure it's the exact same symptoms.
Back to the original issue. A good scan tool, that a decent tech should have, should be able to see signals from the cruise control and +/- gear buttons. First thing I would do is pull up those parameters on the screen and go for a drive, push those buttons, and see if the ECM actually is seeing those signals. For example, even if if the cruise control won't activate the ECM should still be seeing the request. Also are you talking about the cruise not turning on (screen will say something like "cruise control ready"....but you haven't set the speed yet), or is it on on but the speed won't set and hold? The brake switch should not cause the system to no become active or ready, but may very well not let you set a speed and it actually start controlling the speed. Same thing with the +/- gear buttons......not 100% sure but don't know why the brake input signal would cause it not to work or respond.
Anyway, if the ECM sees the signals that means the buttons and circuit to the ECM works and there is something else that is cancelling out those commands. If the ECM does not see those signals I would suspect a bad switch or open circuit. A wiring diagram may show a connector that has the wires for both of those items in it. Go to that connector and see what it looks like. Then with a multimeter or test light check and see if you get any signal in the that wire when you push the buttons (this is just another basic step in the diagnostic procedure that a decent tech should do). From there you just keep tracing the circuit until you find the break.