A coil spring preload system is applied to a parallel-axis differential having a thrust block interposed between ends of axles splined to side gears within the differential. A pair of spring plates are interposed between the thrust block and the side gears, and the spring plates retain preload coil springs arranged in sectors between the thrust block and planetary gears of the differential. The spring plates are retained against rotation relative to the differential housing, preferably by piloting against the planetary gears. U-shaped access openings in the spring plates to allow application of C-clips retaining axle ends within the differential are preferably closed by filler plates having the same thickness as the spring plates.