2001 Dakota Quad Cab Sport 3.9 with Auto Trans 3:55 Posi Rear
Here is what happens driving around town all day and no issues but things change when driving on the highway. After driving several continuous miles at speeds of 60 mph and above when I stop and the 1st time I back up the truck feels like there are flat spots on the rear tires put truck in drive pull forward no issue put truck back into reverse sometimes everything is good and sometimes I get the same flat spot feeling. I know its not the universal joints or the tires anyone have any idea what it could be?

 

This is going to sound a little odd, but check the front upper A-arms to see if they are loose. Second thing to look at would be leaf spring bushings, particularly on the rear shackles.