2001 Chevy Astro shudders like it is stuck in 4WD

Hi DAVE 23. Welcome to the forum. The number one cause of this problem is different tire sizes. Other brands of all-wheel-drive vehicles use a viscous coupler on one of the driveshafts that allows the two shafts to turn at different speeds which happens when cornering. GM doesn't use a viscous coupler so both driveshafts must turn at the same speed just like a 4wd truck. The difference is trucks can be shifted out of 4wd when driving on dry roads but your van is always in 4wd.

The problem can be prevented by buying four new tires at the same time. It is common practice to replace just two when they are worn out, but this vehicle is the exception. You can even run into trouble by buying two new tires, then buying two identical tires a couple of weeks later. The same brand and model of tires from different batches can be different in circumference enough to put undue stress on the transfer case.

Before you condemn the transfer case, inspect the universal joints. They can cause a shudder. While a bearing in the transfer case could be defective, the u-joints are more common. As for the whine, jack up all four wheels and run the van in gear to see if you can duplicate the noise. If you can, you might be able to find the source with a stethoscope. They are available at auto parts stores for about ten dollars. If the drivetrain has to be under load for the whine to occur, there is a tool called a "Chassis Ear" that is very effective. It consists of six microphones that you clip onto suspect points, then you listen with headphones while driving and switching between them. A lot of mechanics have never seen or heard of this tool but most dealerships have them.

Caradiodoc

Dandy. I'd be looking at the u-joints. Look very closely at the four cups on each joint. There is a rubber seal over the needle bearings. Suspect a problem with any that you see reddish-colored rust around that seal. If you don't see that, you almost have to unbolt each shaft so you can feel for smooth movement or binding. Usually by the time they cause a shudder, it will be pretty obvious when you find the tight or sloppy cup.

Caradiodoc

I'd still be suspecting a worn or tight u-joint. The next thing would be to run it on a hoist to see if the shudder can be made to occur.

Caradiodoc