A progressive rate coil is one where the top part of the coil has a lighter spring rate than the rest of the coil. This allows for a softer ride, but once the top part of the coil compresses to the point of its maximum rate, then the lower, stiffer part of the coil begins to compress at its higher rate. Progressive rate coils have been around for awhile, and are not exclusive to Tacoma TRD suspensions. Like 99TRD said, the TRD package supposedly comes with progressive rate springs and Toyota/Bilstein shocks (not true 5100 series Bilstein shocks, which are bigger in diameter). Also, not to be nitpicky, but Tacoma 4x4s/Prerunners do not have "true" coilovers. They're designed more like McPherson struts, except they're not integral to the steering, like a true McPherson strut. They're sort of a cross between a McPherson strut and a true coilover, without the steering link.

Some Prerunners came with the TRD package, while others didn't. From what I understand, MOST TRD packaged Tacomas came with progressive rate springs, Toyota/Bilstein shocks, cool stickers, and the coveted electric rear locker. But as with most things, there are always certain variables to throw one for a loop.

Hope this helps.

Hope this helps.