Hi Richard Morrison. Welcome to the forum. Your description of the shimmy and additional clues, (thank you), is typical of worn inner cv joints. I ran into a rash of them over a four-year period at a very nice dealership, many model years and a wide variety of mileages, but 85,000 seems a little soon.
I figured out some ways to check for this problem but the definitive method involves disassembly and inspection of the joints' housings and internal springs. Nine out of ten on the products involved the passenger side joint. My guess is because the shaft is longer so it goes through a smaller angle change as it rotates and the rollers remain in one smaller area where the wear takes place.
The biggest clue I found was the steering wheel would shimmy most from 2 - 35 mph when turning and accelerating, such as when leaving a parking lot driveway. In my case, the shimmy usually went away when holding a steady speed because there wasn't so much force on the rollers and they could move freely. When they bind in the worn spots of the housing, the half shaft can't change length and angle freely so it pushes on the spindle which is mounted on flexible rubber bushings. When it moves, it tugs on the steering linkage causing the shimmy.
Most GM replacement parts are too. It is much more cost-effective to buy a remanufactured half shaft although you run the risk of getting one that also has a worn inner housing that got overlooked. I suspect that might happen in one out of, ... Oh, ... 25 or so.
I like to know for sure a part is defective before I replace it, but if you don't have the tools to take the joint apart, replacing the shaft is a fast and not REAL expensive test.
Thursday, November 16th, 2017 AT 2:09 PM