sd180sx

Recently my steering wheel has begun to shake at speed with a slight warbling sound from under the car. Below 50mph everything is smooth but at or above 50 the steering wheel shakes in my hand. I started cheap and had all 4 wheels rebalanced and the problem persists. My next thought is wheel bearings beginning to wear. Before I start pulling the car apart and spending extra money I wanted concurrence from others with more experience. Is there another route I should be going or a cheaper trouble shooting step I can take. My car is a 93 S13 on BC coils. All suspension parts are tight, aligned and have less that 10K miles on them so I have not investigated anything else being the culprit.