Vibration/resonance through the floor

Hey Folks,

so bought an '06 2.0 TDi Quattro last friday and it's got a weird resonance almost but not quite vibration through the floor between about 45 and 60 mph, 4th, 5th and 6th gears and it "scrabbles" a bit at anything more than half lock.

Now i've changed the oil and filters this evening, oil was like p!ss and the under tray wasn't the most secure thing ever...

So it's better but still not right, having read a bunch of stuff on here already so checked the inner joints, there's a little bit of slop there and also picked up cheap complete shafts off evil bay are probably not worth the effort so where should I be looking?

Ta much

sorry, should've said, there's been a lot on money spent on this car recently, not by me thankfully

a recon engine and turbo
clutch and dmf
centre prop bearing
4 x discs and pads

although not recent it does have 4 matched premium tyres fitted,

gearbox and diff oils, now there's an idea...

Quick update people, 2 issues found

firstly, car has had the centre prop bearing replaced and obviously gone to great trouble to get the prop balanced too as is stickered up from some where in the midlands...

problem is the exhaust was cut to get into the prop, when it was sleeved back together again the ends weren't butted together adding about an inch to the length of the system, result? tail pipe on the NS was touching the rear bumper causing vibration & noise through the floor, strike one!

second issue, got the wheel balance checked, NSR wheel has a decent buckle in it requiring 70g to get it to balance out even so it's still egg shaped, strike two!

so it's a lot better but still not right

and to add insult to injury the DPF warning came on when coming home from the MOT place, damn car!!