Scary problem with steering

My 89 celica has an intermittant problem that no mechanic can figure out. Symptoms are : Just out of the blue, steering wheel makes sudden, sharp, short jabs pulling to the left, accompanied with load knocking in front and a bad vibration. There is no pattern to it other than it begins when backing off the accelerator.  It sounds like CV joint noise, but apparently the CVs are fine. It can do it on the highway at high speed or turning corners at low speed....its so wierd and scary. It will drive fine for weeks at a time, and then suddenly it will happen for no good reason, and its frightening. One mechanic took a good look under it and said it could possibly have something to do with the tranfer casing/gearbox or shaft in that general area. He shook the shaft and said he felt a bit of up and down movement where there shouldnt have been, but other than that he was stumped.  The car just had a grand spent on it replacing bushes and wheel bearings, but no change. Does anyone have any idea what could be causing this? I havent been able to find anyone with a celica with any similar syptoms and its very distressing. I'm too scared to drive it anymore cuz when it happens it sounds like the axels gonna break in half or something!  Any help would be greatly appreciated.