Welcome to the forum David!
I was going to look up your mower at
Sears Parts for a diagram. But the website is down for maintenance overnight.
Just to make sure I understand, it get's locked in gear after it warms up even with the clutch and brake pedal depressed?
Does it stop moving when the pedal is pressed?
Do the gears ever grind when shifting even with the clutch depressed?
My first thought was maybe the clutch is not releasing the full tension from the belt. That will cause them to be hard to get in and out of gear.