I am with Ken on the tire belt separation. Years ago I had a 1978 Datsun pickup with the same type of problem. It took a while to find the problem. It turned out to be one of the front tires. When you put on the brake the steering wheel shakes back and forth. When you release the brakes it keeps going for a little while until your speed gets down to a certian point. Try and rotate the tires to see if it goes away. If it has happended recently the tread may not have worn enough to tell that there is a belt separation.

Glen