2000 buick ultra lost overdrive, tranny shot? My buddy put scan tool on it and says module bad. He says its under side cover. Fixable or does tranny need complete rebuild?

Tranny needs to come out. You stripped off the splines on the 4th gear clutch hub. Very common problem for the 4t65e until 03 than GM started using a hardened hub. My parents 01 Lesabre just had the same issue. And no it won't set any codes when this happens.