I think a bit of clarification is needed--as matt b suggested jacking the "crossmember" when I think he really meant the control arm. Changing out the springs on the 122-1800-140-160 is simple.
For the front---jack up car and place jack stands under the frame or jack points. Place the floor jack under one lower ball joint and lift the pad so it just touches the ball joint with a slight amount of pressure. Disconnect the sway bar links to the lower control arms. Remove the shock on the side you're working on. Loosen the lower ball joint nut, leaving the nut on a few threads. Use the two hammer method (so you don't rip the ball joint boot with a "fork") striking the ball joint receptacle of the spindle from the side. A few whacks usually does the trick. When the ball joint "pops" remove the nut and lift the upper arm clear. It may now help to loosen the nut on the lower control arm crosshaft so the lower arm can swing down when you slowly lower the jack that had been placed under the lower ball joint. The arm will swing down and the spring will fall out. Installing a lowering spring will be easier than stock as it is shorter. Reverse all the steps and be sure to tighten the A-arm crosshaft nut/bolt with the jack lifting the the arm to ride height level.
For the rear---jack the rear from the diff "banjo" with jacks stands placed under the jack points--or frame rail. Lower the car onto the jack stands but leave the jack under the diff. Disconnect the shocks and limiting straps and then lower the diff till the springs are free (doing both sides at the same time). -- Dave