It could be that it's not a mobility issue b/c of her size, but rather b/c of her confidence levels and muscle imbalances/weaknesses that have built up over time. A year of compensating for her missing leg, and a year of avoiding things, can build up bad habits.

Have you tried PT with her? It might be a good way to not only start to correct some of those habits and give her increased confidence, mobility, and musculature, but also a good bonding experience and help build trust between the two of you.

We have a chihuahua tri (missing front left).  He is hesitant with new hard surfaces if we go someplace new. We tried boots, but he wouldn't walk at all then. He always works it out though. I have no idea about harnesses though. We use a regular collar.  I found this site while I was looking for some kind of jacket that will work for him without getting twisted around.

He is very capable little guy though.He jumps off the couch, the beds, and our side porch.  He goes down to the barn when we go to take care of the chickens.  If we go to a street festival event, he goes on his leash.  He will get tired and a little overwhelmed by the crowd after a while. I just pick him up for a bit and put him back down when my arm gets tired. He goes out with the big dogs (weimaraner and german shepherd) when they go out, and will run all over the 2 acre area they have access to. I think not babying him too much has really helped him be a confident dog. He's 8 and has been a tri for 7 years.  Good luck! Patience will pay off.

zprface said ...He jumps off the couch, the beds, and our side porch. 

Be careful with all that jumping, especially jumping down with only one front leg. Rehab vets we have interviewed described how the extra weight absorbed on remaining limbs causes hairline bone fractures over time, and on more than one occasion we have heard from members reporting injuries caused by jumping.

Oh, it is not encouraged.  Stopping him is the trick!  The porch is only about 18", and he jumps into the nice fluffy weeds . The steps are 4 feet away. and there are only 2! As far as the couch goes, if I'm going to get up and be away for a while, I put him down on the floor. It's when I just walk across the room and I'm coming right back, that he decides he must follow me.  The bed is another story.  He's jumped off of there twice. To be honest, I never, ever expected he would do that something like that. The first time he did it, I didn't see him do it.  I had just walked into the kitchen, and I heard him cruising down the hall.  I thought I must have been wrong about where he had been.  I sat him up on the bed, and sat on the couch across from the bed. There is a really fluffy large dog bed about 2 feet from the bed.  He looked over at me and then just LAUNCHED himself off the bed and landed like a belly flop on the dog bed! He looked like a flying squirrel. He ran down the hall headed for the kitchen, like it was nothing!  He's not allowed up there alone EVER anymore! Never ever!  I don't think he has any idea that he isn't big and 4 legged.