Thank you both for the replies!
I figured it out. For some reason the avatar is in three parts. Once I joined the torso and legs before weighting the clothing, it uploaded just perfectly! Is that Avastar avatar suppose to be in three separate parts (head, middle, lower)? If not, I must've fudged somewheres lol. Previously I was weighting the dress to the legs and torso separately, which was twisting the mesh sideways upon upload.
So excited now. <3 Thank you both again for taking the time to help. :matte-motes-big-grin: