Rear differential rumble?

I purchased a set of front and rear 5.29 gears assembled in sept 2019 from ecgs for my 2016 trd sport automatic Tacoma. I installed the third member and front diff myself and went through the appropriate break in period and changed the fluids after 500 miles and used the recommended gear oil. I have roughly 20k miles on the new gears. I’ve been running 315/75-16 BFG km3. A few days ago I started having a new issue. When going roughly 20-60 MPH if I just feather the accelerator i hear a noise coming from the rear diff that’s a rumbling noise. It sounds like I’m driving over a highway rumble strip. I took it to my local Toyota dealership and the confirmed the noise is from the rear differential but they will not inspect further since it was regeared. I changed the diff fluid yesterday and found nothing more than a normal amount of the metallic sludge on the drain plug magnet. This didn’t fix my problem. Do you have any ideas of what this might be?

 

I'd suspect pinion bearing issue first if it is actually the rear diff, but you won't know for sure until it's inspected. It very well may not be the rear diff so inspect everything.

 

If you find after doing all the inspections as best you can and you’re about to start spending money on your list of “best guess repairs”, please take it to a drive line specialist and have them put the truck on rollers, sensors and computers, pay their 2-3 hour fee and let them tell you exactly what’s going on.