1999 Ford Expedition vibration when AT changes gears???

I have a 1999 Ford Expedition Eddie Bauer Edition with an Automatic Transmission and 125K miles. When the transmission shifts into overdrive when cruising in the city around 45mph I hear and feel a slight vibration during the gear change. It seems to be coming from either the driveshaft or rear differential. I usually only feel it when shifting into overdrive and it seems to come and go depending on how im driving. I was told it was the transmission mount causing the driveshaft to vibrate due to lack of proper line-up??? Any ideas on this? If it helps, my transmission changes gears flawlessly, no slipping, jerking or anything like that. Thanks!!!1999 Ford Expedition vibration when AT changes gears???
Change the mount yourself, you can get trans mounts these days for less than 30 bucks. If that doesn't fix it (which I doubt it will), I'd take it to a transmission shop and have it inspected. You will come out cheaper in the long run doing that rather than just guessing at it.1999 Ford Expedition vibration when AT changes gears???
get the tranny flushed and the filter changed then have them check the u-joints for play or being overly stiff

if that is not it then you need the rear end serviced