transmission slips out of reverse when backingup

i'm a newbie here. always came to this forum to find helpful info and good ideas. finally decided to register and get specific answers if possible. such as why my 74 cj5 slips out of reverse when backing up unless i hold the shifter in reverse.:?

 

check all the mounts first, motor and transfer case.
Make sure the shift stick isn't hitting the body/shifter opening
short of that, remove the top cover and check for worn/bent/broken shift fork or some other mechanical problem: bad poppet ball/spring, shift rail, etc.
no other problems with the trans, noise, shifting etc ?

 

thanks for the reply, no problems with the mounts and no noise of any kind. so my biggest problem is finding the time to pull the top and look for damage.