Why doesn't it stop sometimes when in reverse?

putting it into a gear and hearing a knock indicates that the engine mounts or transmission mounts are broken
naturally if you put it in neutral it will not move as that what neutral is
perhaps the neutral position needs adjusting as it may still be in drive
have the fault codes read to determine any sensor/servo/solenoid faults before spending big money on a transmission

Need more info: is manual shift if so 4sp ,5sp or 6sp speed?
or auto w/overdrive ?
2) wont go into gear or selects reverse gear but acts
like no gear?
3) does forward gears work?
~
4) how's the fluids?

Do an upper intake clean that will clean the throttle body and the Idle Air Control valve. Also clean the mass air flow sensor

sounds like it is time to get the transmission rebuilt. usually when they go out you will only have reverse. It sounds like the oil pump has exploded inside the trans. sorry for the bad news

there is a limp mode but I doubt if that is the problem
It indicates that the torque converter clutch servo/solenoid is faulty
run the fault codes to confirm if that is it or a faulty TCM
check wiring around the transmission

Put the vehicle in Park, disconnect the shifter from the shift lever on the trans, adjust the linkage or cable so the connection is clearly a slip in/on connection and tighten things back up. Hopefully this will solve your non-reverse. As for all the gunk and metal shavings, friend, you better have another tranny standing by in your garage when this one stops working completely.

Either your linkage is off (check by trying to start in neutral.If it doesn't start this is the prob either adjust or sometimes you have to get a new one.

First step here is to have the engine control module scanned for fault codes.

take it to dealer for computer diagnostic check,this sounds like an ECU fault