Drive gear vibration?

I have a 1974 duster with a 3.7l slant six.
Lately I have been experiencing some strange vibration when I am driving.
In drive gear (only) when I have my foot on the break when I'm stopped, something under the hood vibrates irratically, causing the steering wheel, and dash components to shake a little.
This problem only happens in d gear when i'm breaking, all other gears are fine, and my idle speeds are normal.

I have no clue as to what is causing this and was wondering if maybe its a transmition problem, or low oil/fluid/lubrication somewhere?

Any thoughts would help.
Thanks.