1977 280E Automatic Transmission doesn't lock in park ??

My automatic transmission in my 1977 280E does not lock when placed into park. When the car rolls with it in park it makes a clicking sound. I think there is a part on a transmission called a parking "pawl" and I think it jambs into something that acts as the lock. Don't know how it works on this car but I'm guessing that it must not be fully engaging and is the reason for the "clicking" noise if it's partially engaged. Please let me know if there is an easy fix for this. Thanks.