An employee was preparing to remove leaf spring bushings on the front drive axle of a tandem axle tractor. After raising the axle with a hydraulic jack, he placed two jack stands under the rear frame rails of the tractor and placed additional jack stands under the front drive axle. He then repositioned himself at the rear of the tractor where he reached for the release valve of the jack to begin lowering the axle (and frame) onto the jack stands. He was in a kneeling position while using his right hand to operate the release valve. His left hand was caught between the lowering frame rail and a jack stand. His left ring and middle fingertips were crushed, resulting in amputations.