1993 Jeep Grand Cherokee Loud Clunking Noise

I bought a 1993 Jeep Grand Cherokee Limited about a month ago. It has a problem when going high speeds 60mph+ on the freeway. You start hearing a loud clunking noise, almost like the sound of a flat tire, coming from beneath the vehicle. As soon as it starts making the noise, if you pull over to a stop then go again, the noise ends. Then after 5 mins of going at 60mph speeds it start again. Ive had three different mechanics so far look at it, all of them are stumped. When the vehicle is raised, the noise wont happen, only when the vehicle wheels are on the ground. So far the rear end has been replaced, and the transfer case. The U joints are solid, were checking the front end next but still arent very sure that is the issue. I know its hard to say without experiencing/hearing it for yourself, but do you have any ideas?