Howling- Jeep Grand Cherokee

I own a rear-wheel-drive 1999 Jeep Grand Cherokee. I recently had both rear axle seals and bearings replaced. At the same time I had the mechanic check U-joints, shocks, etc. The noise from the failing bearings is gone, but now I have a loud “howl” that gets louder the more I accelerate. If I take my foot off the accelerator, the sound is gone. As soon as I accelerate, the sound returns. It is barely noticeable at speeds less than 35 mph, but the howl gets increasingly louder as the speed increases.

I returned to the mechanic the same day I picked up the Jeep. We went on a road test and I was given a story about gears in the differential and that despite the noise everything was mechanically sound. Even though the sound was not present before the seals and bearings were replaced, the mechanic assured me that nothing is wrong. This howling is giving me a headache! Can anyone suggest a fix?

check the differential fluid