Tire thump when turning

[betabob]

Greetings,

My 98 4WD Explorer starts feeling like the front tires are flat when turning after driving around 18 miles and not before. It has done this predictably three times. The last time was a test drive and I stopped, turned off the ignition and got my car buff neighbor to drive in less than 5 minutes. He put it in reverse first and we drove 5 miles but the problem wouldn't repeat. I suspect the front differential but hate to dig in without certainty. The CV joint boots are intact and there is no clicking noise. The lower ball jooints are worn but the upper ball joints and tie rod ends don't have play. Has anyone experienced a similar problem.