Yep, that will get er' shaking. Rented a Jeep last month in Moab. Same deal. About 40 mph and hit some frost heave in the asphault road and shook like a Mo-Fo until i was almost stopped. Check youtube, there's some good videos on there showing where to investigate areas that cause DW.
There's also some good threads on this forum. Just use the search box up top.
Let us know if you find the cause.