By raising the suspension you alter the steering castor angle. This is shown by the reduced self centering of the steering. This is why there are radius arms with to correct the castor angle or offset bushes for the radius arms.
The vagueness and play could be any number of things as mentioned previously. I'd start with all the track rod ends, especially at the pas box drop arm.
Any steering vibration at approx 50mph I'd check the swivel housing preload.