Sounds like a fun build. But I think you may have caused a problem using the 70s rear axle.

I think the 97 ecm used the speed sensor on the rear axle to control overdrive which the 70s axle doesn't have.

A 91 Dakota ecm would have used a speed sensor on the tail shaft of the transmission. And the 91 Dakota axle would have a rear axle sensor for abs.

Where are you picking up the vehicle speed reading?