Yesterday, my Jeep ran fine. Today, it would not start. I found out it would only start if my foot was on the brake pedal. It runs as long as my foot is on the brake. Once I let go of the brake, it stalls

Hello, hands on inspection is needed. One guess I have is a possible vacuum related concern -- since your brake pedal engages the power brake booster, which is vacuum driven. Since stalling occurs when manipulating engine vacuum, perhaps there's a correlation.

Do not know your skill level but disconnecting the brake booster vacuum hose and blocking it off will determine if the booster has failed internally - - engine then starts and functions normally. ⚠️ DO NOT DRIVE THE VEHICLE WITH BOOSTER HOSE DISCONNECTED!! ⚠️