There are some things to consider with the Boreston Shaft. First, the tolerances on shaft are must better than the OEM unit to where it bolts to the steering column and lower steering shaft. In fact, if you have ever installed a replacement OEM shaft, you will notice how easy the new shaft goes into the lower steering shaft. This is one of the areas that causes vibration that resignates from lower suspension, thru the steering gear, up to the steering wheel. Secondly, there's a vibration dampener that is built into the upper portion of the steering shaft that helps with this issue. Thirdly, while the shaft is more expensive, you do get a 5 year warranty on the shaft-which is more than double of what the OEM has.
The other area(s) that are a source of vibration; first, is the bearing in the tilt wheel assembly area where the assembly pivots. The metal will elongate over time allowing the steering wheel to have movement in the steering assembly. The metal of the assembly is made of a very poor casting that will allow the pins to move freely over time. In fact, I'm surprised that GM didn't machine a bronze bushing in the assembly to help with this issue. Lastly, there's also a upper bearing that holds the shaft in the center of the assembly near the steering wheel that can be a source of the vibration. You will see that some members have had to have the entire steering column replaced. As a result, they found the problem to be fixed.