As evidenced within the data provided, I believe [b̤] and [b] are separate phonemes, as it difficult to ascertain any specific environment in which each occurs to proclaim them as allophones of one phoneme.
Although there are no real minimal pairs provided there are some near minimal pairs such as [bara] ‘large’ and [b̤ari] ‘heavy’, [bap] ‘father’ and [b̤ag] ‘part’, and [bais] ‘twenty-two’ and [b̤əs] ‘buffalo’. So I would assume that the two sounds are distinctive and thus two separate phonemes in complementary distribution.
Within the data provided, the voiceless high vowel [ị] only occurs before a voiceless consonant, whereas its voiced counterpart [i] occurs elsewhere, thus we can determine that the voiced high vowel [i] becomes voiceless preceding a voiceless consonant.
The voiceless high vowel [ụ] however, only occurs following a voiceless consonant and its voiced counterpart [u] occurs elsewhere.
On turning to the task of writing a general rule however, I found that I could write one rule for both vowel pairs by re-stating the environment in which the change occurs, as between two voiceless consonants.
The general rule therefore would be: the high vowels [i] and [u] become voiceless between voiceless consonants.