Watch the first 3 minutes of this video. Really you should watch the whole thing, but the first 3 minutes will show you the effect of lift on drive shaft angles. The angles are what are causing the low speed vibration. In the video we are demonstrating how a 3" lift affects angles, I know yours is 2.5", but the overall effect on angles in going to be very similar.
As far as lift not needing an SYE and double cardan shaft for lifts under 3.5 or 2.5 or whatever number. I disagree. I of course have a uniquely biased perspective. But that perspective is not based on selling drive shafts so much as it is on my understanding of ideal operating angles of drive shafts and being a bit of a perfectionist in my definition of what "works". That is really the key, defining what it means to you for something to "work". If all that means is that when you press on the gas the Jeep moves forward it is pretty easy to accomplish that goal. If you are more concerned about getting the best performance out of your driveline, the smoothest operation and the longest wear life, the SYE and double cardan shaft is recommended. Generally though, regardless of lift height, I think it is best to prescribe treatment based on the symptoms. If someone has a 3" lift but tells me that they have no low speed vibrations from their stock shaft, I don't see too much of a problem and don't like telling them they "need" an SYE. But if someone does have those low speed vibrations I think the correct way to fix them is with the SYE and double cardan shaft.
