I don't think there's a specific name for the phenomenon you're describing. However, these changes are often regional and are called mergers
A merger is a phonological change where two (different) phonemes merge and become indistinguishable.
All the cases you've mentioned are mergers.
Your first case is an example of fell-fail merger.
It's a merger of /ɛ/ (as in 'fell') and /eɪ/ (as in 'fail') before /l/.
According to Wikipedia:
It occurs in some varieties of Southern American English making fell and fail homophones. In addition to North Carolina and Texas, these mergers are found sporadically in other Southern states and in the Midwest and West
The second one is an example of fill-feel merger. It's a merger of /ɪ/ (as in 'fill') and /i:/ (as in 'feel') before /l/.
The fill–feel merger is a conditioned merger of the vowels /ɪ/ (as in 'fill') and /iː/ (as in 'feel') before /l/ that occurs in some accents. In Europe, it is commonly found in Estuary English. Otherwise it is typical of certain accents of American English. The heaviest concentration of the merger is found in, but not necessarily confined to Southern American English: in North Carolina, eastern Tennessee, northern Alabama, Mississippi, Louisiana (but not New Orleans), and west-central Texas.
The third one is full-fool merger. It's a merger of /ʊ/ (as in 'full') and /u:/ (as in 'fool') before /l/.
The full–fool merger is a conditioned merger of /ʊ/ (as in 'full') and /uː/ (as in 'fool') before /l/, making pairs like pull/pool and full/fool homophones. The main concentration of the pull–pool merger is in Western Pennsylvania English, centered around Pittsburgh. The merger is less consistently but still noticeably present in some speakers of surrounding Midland American English... (more details at Wikipedia).
There's also salary-celery merger which makes celery and salary homophones.
Head over to Wikipedia for more details.
Also, as John Lawler said in his comment, sonorants/resonants affect preceding vowels. For example, vowels are often nasalised before nasals, and r-coloured before R (AmE)
I might call this particular case pre-L laxing; pre-L means before L and laxing means shortening.