Terrible Shelby, just terrible...
Well, if your fluid level is good, then it's obviously not the same thing, but that's exactly what my car was doing before I checked the fluid and was a quart low - sounded like something was binding in the tranny and it was jerking on me, especially in reverse. Very scary.
However, granted though I don't know these trannys as well as some others I've worked on, I thought the sprags were used in tranny braking - as in non-overdrive usage, but weren't actually used to transfer power. Maybe I'm wrong though. It more sounds part of the piston apply function has failed - either a seal has blown that's causing a piston not to fully engage, or a piston return spring is jammed/malfunctioning. I'd think that if the sprag had failed, it'd just "zing" on ya. Also, you'd have had to be drivin' it like a mega-***** to get the sprag to blow, wouldn't ya?
What does ChrycoPsycho have to say about all this? Again, if I were nearby, I'd gladly help tearing it apart with ya - I love tranny's.
Jeff