Chev 2003 silverado shudder from rear transmission

I have a chev 2003 silverado with ~150k miles. between 15-20 mph there is a shudder that comes from the rear transmission. It gets worse when there is a load in the truck. It always occurs and only between the stated miles. I have had the differentials checked and they are ok.

Your truck has two transmissions? That is a new one to me.

Anyway, have you checked the u-joints in the driveshaft?

Yes you are right. I should have said the transmission. And I did have the U-joints checked and one was replaced about 30K miles. They were checked again just recently and they are okay. When I replaced the ujoint it made the shudder less but now it is back to the same as before.

PS I am not mechanically inclined and do not know that much about cars.

So, to be clear, you have 120k miles on the replacement U joint? Or is it 30k miles since the repair?

30K since the repair.

Have your mechanic check the splines on the driveshaft, where it links to the u-joint.
He should have done this when the u-joints were checked, but he may have missed this.

Thank You! I will suggest this to my mechanic and take it from there.

The shudder might be coming from the driveshaft slip yoke in the transmission. These slip yokes can bind where it doesn’t allow the driveshaft to slide as the rear axle articulates up and down. This can cause a popping/snapping noise or a shudder.

Have the driveshaft removed from the transmission, and lube up the splines on the slip yoke with moly grease to see if this gets rid of the shudder.