You don't need to raise it high to get the leg out. You lift it out the top, not drop it out the bottom.
That's correct. You'll have to support the frame near the leg you are replacing or just hook it up to the truck. Once the holding bolts are removed, you can pull the leg out through the door hatch. Then just reverse the process to install the new one. The more challenging part is lining up the motor mechanism with the leg screw so they engage properly... otherwise it's pretty straightforward.
I just did this two Sundays ago. One tip for you. Even with the screws removed, the steel clamps will still be tight. A block of wood and a mallet helped initially, but ultimately what worked best was a ratchet strap around a clamp and connected to the truckís trailer hitch. Just ratchet a little at a time till you can pull the leg out from between the clamps.