904 or rear axle runs jerky help!

That's a good guess C. Tires can do some crazy stuff when a belt breaks.

If it's not a tire I'd suspect the rearend more than the trans. Maybe a broken tooth on a ring gear. Have you taken it up to speed? If so how does it do, any bad noises? If it's a rearend you could probably diagnose that by jacking it up and setting it on stands and turn the wheels by hand to feel if the rearend has any tight spots. I've even seen brakes come apart and jam up against the drum and cause it to jerk, but that usually causes quite a bit of noise also.