I can't confirm the return line but if you run the rear heater and adjust the temperature control the cooler of the two lines should be the return.
An I/R gun would make it easier.
Try removing the rear drive shaft and see if it still does it.
If it still does, reconnect the rear and remove the front.
With the drive shaft removed you can check the u-joints better as well.
I had a vibration that I couldn't figure out.
U joints on front and rear drive shaft seemed ok.
I disconnected the rear driveshaft and drove it in 4wd.
I only had to go about 100 feet when it was obvious it was the front drivesahft.
For some reason with the rear attached it reduced the front...
I tow a boat/trailer that weighs 3200 lbs.
175,000 miles 3.4 AT 4WD
I added a trans cooler and that's it.
I tow on flat land with some small hills.
Trailer has surge brakes.
Stock type shocks and springs.