Tedster,
You can mix and match suspension components for the 1996 - 2002 model years until the sun goes down. For the most part, all of the hardware is interchangable. Things such as ABS sensors, traction control, lockers, etc, changed over the years, but spring seats and shock mounts are common.

If you are interested in doing this, the springs were around $220 shipped, from Jay Marks (Champion) Toyota in Texas. I dealt with Mike C. Simply specify that want all four coils for a '99 4WD, auto, V6 4Runner. Even easier, supply these parts numbers: Front right: 48131-35400, Front left: 48131-35350, Rear (both): 48231-35210. I don't think you could purchase these new elsewhere (other than from a toyota dealer). If you want a used set of '99 coils, contact drag1 on this forum.

Yes, the rear axle needs to be gently lowered with a floor jack in order to swap the springs. The frame of the truck should be sitting on jack stands. Then you undo the lower shock mounting bolt and unbolt both sides of the sway bar end links. Slip a floor jack under the differential housing, slide the shocks off the lower mounts, and then gently let the axle down. I also undid the brake line hard mounting on the axle tube to give a bit more wiggle room. Then its a matter of having someone push down on the brake drum while you manuever the coil spring out.

With you rough riding post from earlier, I'd recommend getting the '01-'02 bumps stops and isolator cones from a dealer. The bump stops will be shorter than your current ones and reside on the frame, just over either end of the axle tube. These determine the ultimate amount of upwards travel your rear axle will achieve. The isolator cones are soft rubber cones that plop into the tops of the rear coil springs and dampen the movement of the truck as you go through a big dip in the road or carry a heavy load. Take a look at your rear springs - my guess is that your truck does not have the cones, and that the bumps stops are of such a length that they are within a couple of inches of your axle housing.

Let me know what you decide to do.

Andreas