Sultan - I disagree, I have encountered full on ground resonance in a Wessex and the early stages in Sea King - there is enough time to do something about it but you need to have been trained to recognise and deal with it.
As soon as they landed and experienced the 'padding' that precedes GR, they should have lifted to the hover - if the oscillations stop, that is when you consider a running landing or a low hover throttle chop.
How much GR awareness is there amongst the Blackhawk users? If it is a very unusual occurrence on that aircraft then it is understandable why they didn't recognise it and act quicker.
From the report:
13. The pilots were attempting to follow the NATOPS emergency procedure by pulling the PCLs for .. Unusual Vibrations on Deck", but were unable to do so based on the violent shaking of the aircraft and the centrifugal forces resulting from the yawing motion of LT 616. The PCLs were most likely not secured prior to the aircraft rolling off the flight deck. [Findings of Fact 63, 93]
Apparently pulling pitch is not an option per the NATOPS. As stated the procedure could not be accomplished due to excessive vibration. This does highlight the benefits of collective mounted throttles like on Bells.
Bottom line there should not have been a single unenunciated failure mode that could lead to ground resonance. The question now is why did NAVAIR allow this design to proceed?