I just got the truck running today, drove it around the block, and when I got up to around 30-35 I got a horrible shake from the wheel, so bad that it would be impossible to drive any faster or maintain speed and stay in my lane. My dad thinks it is a tire/wheel issue. Its an 89 K5 with 3/4s from an 82 k20 and ~1/3-1/4 tread 38.5 Gumbo mudders, all of which is a recent addition. Edit-3/17/07 Took it to a shop, aligned front end, balanced and trued the tires. It needed a passengers side tierod end, but that was all. It was working fine until this afternoon when I went wheeling, on my way between mudholes it came back. I described it wrong above the shake is everywhere but the steering, It is like the whole truck is shaking. I took some rear shims out when I swapped in the 3/4 tons, and now there is almost no angle between the driveshaft and pinon yoke. It is not shaking often as it was before. Any ideas?