2008 GMC Yukon 5.3L engine vibration at low speeds with cylinder deactivation

I had a thought this morning. One of the main things which goes wrong with the cylinder deactivation as GM does it in the LS engines is one or more lifters will go bad very often once there's a few miles on the vehicle. When the lifters start to wear out (I believe there is only a different lifter on the intake side), they will start to lose pressure very easily and will do cylinder deactivation on their own. Later when they are completely worn out, you'll get the problems at pretty much any RPM. This is a very typical issue with the cylinder deactivation as GM implements it.

What could be happening in your case is, as you are slowing down in your Yukon, the oil pressure will start to drop, which will cause the lifter to not be fully pressurized. As you step on the throttle again and start to accelerate, the oil pressure goes back up and the lifter will get back up to its proper "pumped up" height, allowing the intake valve to open as it should.

If this is the problem, in order to fix it, you have to replace the lifters at a minimum on the intake side (it makes sense to replace both, though, since you're already in there). To get to them, the heads have to come off (replacing both lifters and trays), so it's a bit of work.