2003 Ford Explorer Shutter or vibration

My Ford Explorer runs fine in lower gears and on level ground. When I am in 4th gear or OD traveling up hills
I have a shutter or vibration that feel like it's possiby in the drive train. The vibration starts around 45-55 mph. And last for 15 seconds. It starts our soft then gets wrose and the you feel a slight thump and it stops for 5 seconds then starts all over again. It does this as long as long as I'm going up a hill or grade. ( The vibratinon is not felt in the steering wheel) I've had the transmission serviced on schedule, just recently repleced the transfer case. The dealer said it was my torque converter. I took it to a very repitible transmission shop and they said the torque c checked out ok. Please help.