After I did a 50mm lift I had so many wobbles & shakes from the front and rear of my MN I didn't know where to start. The steering was doing what you described as well, it was bloody terrible.
So, I tried wedges, weight changes, Tail shaft spacers, gear box & engine mount changes, Turning the tail shaft center carrier bearing & wheel alignments etc.
Now, I did I end up getting these wobbles & shakes down to an acceptable minimum, but I could never get them to completely disappear..
That is until I put a straight single piece tail shaft in it. Soon as i did that she was back to normal with not one wobble or vibration from the steering or transmission. It's the best thing I ever did.
I have had the straight tail shaft for over 12 months now & I have had both rear wheels off the ground at the same time in the scrub a few times & it has never touched the cross-member, Although I have had the cross member modified slightly to give an extra 25mm clearance, but IMO it was unnecessary.
EDIT: Might pay to check the welds on your rear axle housing spring seats as well. We thought my MN chassis was bent left to right (not up & down) because the rear was way out of alignment with the front & no one could get it aligned true. It turned out that the weld on the left hand rear spring seat was not done correctly at the factory. It was positioned incorrectly and out by 10mm, it was also bent downward at the front. How it got through factory inspection like that defies belief because it would have been near impossible getting that rear spring on there. Although it did not make the MN wobble or shake, but while you were driving it just felt like the rear end was trying to overtake the front end.