The rear shackles can be a little tricky to remove if the bolts have rusted to the metal shims inside the bushings, but there are some things you can do to make it a little easier.
Before you're going to remove them, spray some kind of penetrant (Liquid Wrench, PB Blaster, even WD-40) into the space between the rusty bolt and metal sleeve.
Use a breaker bar. A 1/2" ratchet with a pipe isn't nearly as good, but is still better than trying to do this kind of job with a 3/8" handle.
A trick that I used when popping loose the lower bolt (the upper bolts in the frame don't seem to rust to the sleeve as easily) was to remove the top bolt from the shackle, then put it just below the frame, reinsert the top bolt, then drop the whole frame onto the whole thing, which twisted the nut/tab on the lower bolt, and popped it loose. This will only work if the tab is in the usual position for a fully tightened factory bolt and the shackles are in good enough condition to handle the weight and torque the bolt/tab.
If they are severely rusted together and there's just nothing you can do, you can use a hacksaw/grinder/cutoff wheel and slice the shackle to bits, then saw off either end of the bolt, drill the rubber bushing, and push/pry it out along with the metal bushing sleeve, and replace it with polyurethane bushings.
