Forbidden Journey in USH especially is very tame on the jerkiness. Simpsons, Despicable Me, Transformers and even Studio Tour display more of a jerky quality. FJ is more about smooth, precise swaying about, creating the sensation of flight rather than ground motion like those other attractions (and Indy). Of course, you are seated fairly upright and are pushed onto your backside (swiftly, but quickly) more times than one. The best way I can describe it is being like an inverted coaster (Silver Bullet at Knotts for example) crossed with the smoothness of Soarin at DCA.
What I would recommend is sitting on the test seat outside the ride entrance. That should give you an instant indication for how far your back will be from the backrest. If you can pull down the restraint and it locks you, and pins you comfortably to where your body won't be experiencing any sliding back and forth, then you should be good to ride. Also take TM advice, as they've sure as hell ridden it more than I have.
What I would recommend is sitting on the test seat outside the ride entrance. That should give you an instant indication for how far your back will be from the backrest. If you can pull down the restraint and it locks you, and pins you comfortably to where your body won't be experiencing any sliding back and forth, then you should be good to ride. Also take TM advice, as they've sure as hell ridden it more than I have.