Weird shutter at low speed and turning?

I'm getting a shutter when I'm at a low speed and turning. At first, I thought it was u-joint in front, but they seem fine. I jacked rear up and put it in gear and no noise from rear differential. Only happens after I've been driving 10 15 miles. When I'm cruising it doesn't do it. Only from a stop making a turn. Get above 10-15 MPH goes away. Changed rear differential fluid out of caution. Fluid looked as expected for 50000. Shakes whole truck.