Catalysis of the reaction: 2'-deoxyribonucleoside diphosphate + thioredoxin disulfide + H2O = ribonucleoside diphosphate + thioredoxin. Thioredoxin disulfide is the oxidized form of thioredoxin.
2\'-deoxyribonucleoside-diphosphate:thioredoxin-disulfide 2\'-oxidoreductase activity