You may have a seized or stiff Universal Joint in your rear propshaft (assuming this happens when you're in 2WD). Mine went and it set up some nasty vibrations at about 30-40 mph.
UJs are easy and cheap to get. Get two and replace both at the same time. Often if one fails, the off-balance running causes the other one to wear out, plus that if one goes, the other one is probably the same age and so should be done as well.
Be careful not to fit the UJ the wrong way round. Whilst it will fit and work perfectly, the grease nipple is only on one side, so get it wrong and you won't be able to grease the UJ (you may recognise the voice of experience here).
However, UJ noise usually occurs first as a clonk when you change gear, and tends to lessen if you coast along with the clutch disengaged. It may a wheel-bearing or something else, but I can't really comment here....
Jack
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