1999 Ford Explorer Unexplained Smoke at high speeds

There is no oil leak in the Explorer but any time it is driven over 60 miles an hour it drips tranny fluid or oil on the exhaust and causes it to smoke. We have had it looked at and worked on and every person we have asked doesn't have a clue, including the mechanic who fixed the recall at the Ford dealership. The recall was for something that could cause the engine to catch fire but the Ford mechanic said the part he replaced has nothing to do with smoking or leaking. At city speed limits up to 45 miles an hour the vehicle works great and has been completely dependable but limits what we can do because of this smoking problem. We are at a loss what to do because as I said we have asked more than a few mechanics including a Ford mechanic and they seem clueless. There has to be a reason for this do you have ANY idea what it could be. Thank you for your time.