St2 gearbox removal

[St2problems]

I have a focus st2 60plate. When dropping the gearbox, does the subframe have to be dropped? And engine supported?
My car decided to stop dead pulling off from a junction the other day and spit gearbox oil all over the floor! Tow truck came winched it on the back and the front wheels were locked but when it got back to mine the car rolled off the back and I had to reverse onto my drive? What would of coursed the front wheels to lock on like that then free up?
Is it new gearbox and clutch job?

Thanks