This may be kind of tough to explain by typing but....... just installed 3" TC springs w/ 4" blocks on '90 K5. I have a high frequency (fast) vibration at high speeds but I'm pretty sure (99%) this is from my U-joints. I am changing them today. However, at low speeds, and I mean 10 mph there is a low frequency vibration (really more of a "shake" than vibration). It feels like when you just begin to roll and have VERY aggressive knobby tires (my tires are new Firestone LeMans (on/off road) and are NOT aggressive at all). I can't figure it out. If the tires were out of balance it would not be noticeable at this low speed, and this goes away when you get above 20 or so mph. Could bad U-joints be causing this type of "shaking" at such a low speed? Everything with my lift has gone great and the truck rides very well other than this. If I need to provide more info. let me know and I certainly appreciate any input. Thanks.