vibration

i have a 2001 model mitsubishi pajero 3,500 gls auto 4wd which has done 140,000 km.
at 100 km/h, a vibration and noise like a helicopter rotor starts when my foot is on the gas, but stops when i take my foot off. it makes no difference if in 4wd or 2wd.
it continues at any speed above 100 km/h, but at lower speeds it dissappears.
i have had it back to the main dealer, who said tyres, but i changed them and its still the same. i suspect universal joints, but when i checked them, they seemed ok, but im not sure how to spot signs of wear on the u/j's.
any help or info would be greatly appreciated.

thanks in advance.