Success! Took the propshaft off again, did another check of the unijoints and turns out, one bearing cap was out of place. Circlip was still in place but not sitting in its groove. Pulled it apart and sure enough, one of the needle bearings had come loose and was sitting in the base of the cap. Reassembled it properly with plenty of grease, put the propshaft back in and viola, no more vibration. :banana:
Thanks to everyone for their help, much appreciated :BigThumb:

