Hi C.,
My grandson is also 3 1/2, autistic, and is suspected to be challenged by Asperger's. We are fortunate that he his potty trained, except for night-time, although you never can predict when he might take off his clothes, pull down his pants, or pee on a tree outside. Aren't these kids interesting?
These kids for sure pick up on and react to the energy and stress around them. What's the saying?... Kids, dogs, and bees can smell fear. Our kids are doing the best they can to control their environment, and where they pee and where they poop is an important last hold-out for them.
As hard and frustrating as it is, don't react to poop or pee in a negative way. As much as possible ignore offering any reaction to what is not working, and over-celebrate what is working.
Share an ongoing conversation about how cool it is to go on the potty, and allow her to watch you in the process. Celebrate others in the house going potty in the toilet. Don't force, but keep the invitation coming.
Find a way to make it a game. Look for an interest or stim she might have and associate it somehow with the toileting process. Back off at the first sign of resistance, and allow it to be a process. The more you insist, the more she will resist, so remain calm. These kids are so sensory sensitive that the noise of the whole bathroom thing can be overwhelming for them. Summer will help, too, when she can run around with little or no clothes on.
Gut related issues contribute to all their symptoms, so if you haven't already, consider adjusting her diet to GF/CF. I highly recommend Jenny McCarthy's books, and I just read a great free (free is good) downloadable book (as if we have time to read!) that explains the food/autism relationship. You can access it at: http://www.scribd.com/doc/12842657/Gut-and-Psychology-Syn...
I would be happy to imagine more possibilities with you if you want to email me at ____@____.com can then determine best how to connect.
Take a deep breath. This is a challenging hurdle to clear. It seems so important we collaborate and ask. We are blessed (gulp) to have these kids who seem to be here to tell us how broken our environment is. We have to link arms and apply what they are teaching us.
Hope that helped.
R. (Grandma Tutu)