300cd diff noise?

[BenzTurbo]

my 83 300 cd has been making a whining/ grinding noise for a few months. I thought it may have been one of my cv joints because the dust boot was torn. I changed both today hoping for the noise to be gone and it made no difference. When i had the cover off the diff to remove the C clips, i saw no metal shavings and it had plenty of oil. It even has new oil now. The center carrier bearing is new as well. I am 99% sure it is the diff making the noise. I'm sure some of you will flip but i put the back of my car on jack stands, blocked the front tires and rigged my linkage to keep the car going around 30mph on the speedometer. That is where i notice it driving and also started hearing it up in the air. When under the rear, its very loud behind the diff when driving, it seems to get louder and change tone depending on load. There is a 1985 300sd at my local PNP that i was planning to get the 2.88 diff out of to swap. what do you think the noise is? will swapping the diff solve the problem? Thank you!

benzturbo's 300cd diff noise - YouTube