Braking feedback in brake pedal?

The description sounds like the ABS is activating. At the mileage and year you have the common reason for this that doesn't set any codes is rust jacking at the wheel speed sensors. The rust forms between the sensor mounting tab and the hub and slowly pulls the sensor farther from the tone ring. As it does that the signal gets weaker and as the truck slows down that weaker signal drops out and the ABS thinks that tire has stopped rolling. It then activates and reduces the braking on it so it can start rolling again, however the missing signal doesn't return so it stays active until you stop. The result is a whirring or buzzing as you apply the brakes. It is very common on GM vehicles. There are a couple solutions to it, IF you can get the bolt out of the hub and get the sensor out without damaging it, you plug the hole and scrape or otherwise remove the rust. Then apply some grease and put the sensor back in and you should be done. The other involves trying the above, then either the bolt breaks of, or the sensor breaks and those usually end up with a replaced hub.
https://www.2carpros.com/articles/how-to-replace-an-abs-wheel-speed-sensor
Shows the style of sensor but yours goes into the hub not the backing plate.
This video shows it pretty well.
https://www.youtube.com/watch?v=4fyjZB0gWIw
Normally I would say to use a scan tool to test the sensors and go and clean the bad sensor, however as you have one already triggering you should just clean all of them so you don't need to repeat it in another few weeks.

If you have access to a scan tool that can read live data you could look at the wheel speed sensors and see which is the problem, but even then I would clean them all.