This pic was taken from my basement, looking up to the first floor. The drain is attached to a fiberglass walk in shower pan. It looks like it could be unscrewed but I can't see how I'd get a tool into that tight space. Any ideas?

The threaded part of the drain (right up there against the fiberglass) was installed prior to the shower unit being placed in it's current location. You cannot remove it without removing the shower surround since the PVC nut it tight against other framing.

However, the 2" PVC drain can be removed - although you'll likely have to cut it out to remove it. Start by removing the metal drain strainer cover from the shower side. This style drain uses a rubber expansion gasket to make a tight seal against the 2" PVC drain. When the strainer is removed you will see a series of notches around the perimeter. When new, this drain came with a metal tab used as a tool to tighten the rubber gasket. It's a strip of metal about 2-1/2" long by 1" wide with a slot in it for a large screwdriver. Doubtful that you still have it around so you will either have to try to rotate the fitting by using two screwdrivers or go buy another PVC drain and use the tool that is inside it.

However you do it, once it makes a couple of rotations it will be loose enough so that the 2" PVC can be removed from below. Cut the 2" PVC and remove it. To replace, just get a 2" PVC straight connector and chemically weld it in.

Now that I have said all that I guess I should ask why you want to remove this?

I would use my ossilating saw to enlarge the area enough to be able to use a basin wrench.

if I'm replacing the shower pan, I'd cut the pipe, cut the pan, and cut the wasted time.

Forgot to mention I'm replacing the pan. It's a Jacuzzi 60" pan that's in good shape so I'd like to take it out in one piece. Thanks for the replies, I'll let you know how it goes.

Then just cut the 2" pipe and take the pan out from above.

Got it off! I unscrewed the round piece of plastic that's used to hold the rubber gasket in place with a couple putty knives (they were closer than two screwdrivers lol). Then i just lifted up the entire unit - very easy.