My 89 K5 has a left to right shake in the rear when I go over 55mph. The weird thing is its different when it's a cold start versus a warm start. If I get above 63mph it seems to stabilize. I usually don't drive on the highway with it but this week my primary vehicle is in the shop. I've added new shocks, new stabilizer, bearings repacked, body bolts checked, tranny checked, u-joints checked. I guess it could be the tires/rims? I have plans to get some SSR's once I gather the funds.