Dude,
A vibration at that road speed and engine revs should be constant,
not this come and go every two seconds as you describe.
Doctor says:
a) ride at same speed in different gear and still vibrates = rims and suspensions issue
b) ride at same engine revs in different gear and still vibrates = engine issue
c) specific combination of engine revs and road speed it's a drive line issue, which includes drive shaft, final drive, axles, bearings etc.
My initial thoughts are it is a drive line issue.
Coming so close to having new tyres fitted is obviously suspicious though.
I would check, in this order:
# Everything wheel fitter's touched is tight
# Spacers in the correct position or missing - are the wheels aligned correctly with no lateral movement?
(though I don't know if it's physically possible to put it all together wrong)
# Even go so far as to check that there is oil in the final drive and correct spline alignment of hub and rear rim.
# Check all wheel bearings.
# Remove any balance weights on the rims.
# Rotate the tyres on the rims (as previously suggested).
Hope that helps & keep us informed