Zachc, did you ever get this resolved? Saw this earlier but I was at work and did not have the information at hand. According to the workshop manual, error 90 means "forward proportional valve in trouble", which makes sense given the symptoms you describe above. According to the manual, signals from the pedal are transmitted to the ECU which controls the two proportional valves, which in turn control the HST swash plate which controls forward/reverse via the HST variable displacement pump. This one is going to need to go in to a Kubota shop for repair I am afraid.
So far as I know (haven't checked in several years) Kubota does not provide digital downloads for manuals. I ordered mine online from a dealer like any other tractor part. For example, Coleman Equipment in the west, or Messicks in the east.