The /ʊ/ vowel is usually spelled <oo>, <u>, and <oul> in should. The /əʊ/ diphthong is spelled <oCe>, <oa> or <ow>. The computer also found the pair luring/lowering, but I have taken that out since the /ʊə/ diphthong in luring is best treated as a distinct phoneme, and some speakers would use a palatal glide in any case.
This is a contrast between a short vowel and a diphthong, fairly close in their starting positions in the mouth. There is some likelihood of confusion though not associated with any one group of learners.
The density figure is 1.48%. The set makes 18 semantic contrasts giving a loading of 62%.
brook broke bull bole bulls boles cook coke cooked coked cooking coking cooks cokes cooks coax could code crook croak crooked croaked crooking croaking crooks croaks full foal good goad goods goads gulden golden hood hoed hooks hoax pull pole pulled polled pulling polling pulls poles putsch poach putsches poaches should showed stood stowed took toque wood woad
John Higgins, Shaftesbury, December 2009