Ford Repair: wierd powertrain noise, flexplate, vehicle speed

Question
95 Ford Exporer, newly rebuilt transmission, wierd noise just started, noise gradually increases in direct ratio to vehicle speed (regardless of engine RPM and which gear is engaged), noise is the same if put in neutral while vehicle is moving.  Tires look fine all the way around.  No problem shifting or putting in 4WD and back.  My guess is differential or transfer case (I hope I am wrong!) Any ideas on how I can diagnose the cause would be much appreciated.

Answer
Norm-

  Well, if the sound occurs in neutral you can just about rule out the transmission entirely.  So that only leaves a few other possibilities.  Two you've already mentioned...the transfer case and/or differential.  Another possibility would be some sort of friction between the flexplate (or flywheel, depending on whether or not the car's a manual or automatic).  This isn't unheard of after remounting a transmission.  Either way I would have a shop that specializes in transmission and drivetrains take a look at it.  My guess is that something's either worn out or there's a source of friction somewhere in the drivetrain.  Hope this helps.

Steve