Well I can't decide if it is rear end noise or ujoints....it starts at about 10mph. ..and gets louder with speed ..it can let off the gas and it quiets down some and when I put foot back on the pedal it's louder....here is a video. If you can get passed the interior rattle. You can hear a humming /clicking sound...sounds like rear passenger side to me.
https://youtu.be/xit7XlkZR2g

 

Sounds just like mine did when two teeth broke off of the pinion gear.



You could check for play in the U-joints and if all seems tight with the joints then I suggest you jack stand the Blazer as I did and then use a mechanics stethoscope to listen at the rear differential cover for the sound in the video, if it's coming from the diff. then pull the cover and look for broken teeth in the bottom of the diff. housing, also inspect the ring and pinion teeth for damage by turning the rear wheel by hand which will rotate the ring and pinion gears.



I remember that project like it was yesterday



The ole switcharoo, broken toothed open diff. to a boneyard locker rearend.

 

What I was afraid of