i think its an unbalanced tail shaft, on my vl (yeah its not a skyline) i had a 2 peice tailshaft, wheels were ballanced and alligned propley, coudlnt work it out, but it would be find till i did 160 and it would vibrate like hell. i replaced the center berring thinking that was ruined, didnt do anything.
my car was so bad once backing off at 240, u would think it was going to drop the box and send the engine outa its mounts.
i then got the tailshaft ballanced and everything was fine.
have you hit anything under the car? the slightest knock on a tailshaft can put it outa ballance.