Hey mate,
If the engine revved to full revs smoothly no vibration and then at 1500 rpm in gear I would be thinking it could be shaft/prop related,even something wrapped around the prop or shaft.
Not sure if you have shafts or legs but guessing shafts, I had vibration on my old 35 Bertie and it ended up being a misalignment of the shaft to gearbox coupling which an adjustment to the rear engine mounts rectified.
Good luck with it,
Paddy