First let me say that there are guys here that know a whole lot more about tranny's than me (they have been coaching me for about the last 6 mos).
It sounds like either a shift linkage or internal fork alignment issue. Start with the shift linkage, make sure all the allen head nuts are tight and that your bushings are not wore out. Check the Pelican site for a tech article on shift linkage bushings....pretty easy stuff.
It might be possible that a bad trans or motor mount could contribute to mis-alignment when under load. So I would inspect those too.
keep bumping this thread and see if you get any other suggestions.
good luck