1994 Chevy Camaro rear differential trouble?

I have a 94 Camaro 3.4L 6cyl rwd w/ 119k. When I reach about 45 and the rpm kicks down less than 2000 the car shakes. When I press on the accelerator, the shaking goes away. Any ideas? The shaking is rhythmic. Seemed like something in the drivetrain. A mechanic drove it and told me it was my rear differential, but I thought that was a pretty big assumption without popping the hood.