Yep possibly the worst explanation.
What type of box do you have. Is it a rod change or a remote change. do you know the diference?
Well basically if its a rod change you will see two rods. If it is this one then there are a couple of things to look at. Either its the rubber mounts that are on the gear shifter mechanism under the car or its the pins in the linkage that are worn. This is a kind of metal pipe thing that links the rods to the gear box. It has two pins, one on the rod and one on the box but they wear out the linkage so check them.
If its a remote box then when you look under the car you will see a big alloy thing. using your words)
If it is this type of box the main bit to check is the is the large rubber mount at the rear of the remote under the floor. While you are under there put some grease in the grease nipples that are on the remote.
Regardles of the type of box you should also check the bolts that secure it to the floor as one or more could have come loose. Other than that i'm not sure but hope i have helped.