[ImperialCrown]

Welcome to Allpar. Durango is a Mopar, so this topic may be moved to the truck section.
If the truck is fine up to 40 mph and then the shudder starts, it sounds like tires or drivetrain. Either an imbalance or failing driveshaft joint condition?
Further diagnosis is needed. Is the shudder felt more in the floor and seat than in the steering wheel? Does it vary with road speed or acceleration and coast? Does it feel up/down or side-to-side at all?
Shudder 'frequency' is important. A shake may be slower than a shudder. A rapid shudder may be driveshaft related as the driveshaft turns at about 3 times the tire speed. If the shake is about once every tire revolution (still pretty fast at 40 mph), it may be tire/rim imbalance or a damaged tire belt.

 

[valiant67]

The best tests to elimiate tire issues are to rotate the tires or temporarily run them with 5 more and less PSI. If the problem changes, it is tires.
(moved to general tech support for more visibility since this is Mopar related)

 

[KOG]

Speed is suspiciously close to torque converter lockup. Try running it in "3" rather than drive and if the problem goes away you've got a handle on what it is.