I'd say about a 10% chance. I took a quick grid of the worst four teams in each league against the four playoff teams in each league (so HOU/CHC/SD/FLA vs PHI/MIL/ARI/STL and MIN/SEA/BAL/KC vs NYY/TEX/DET/TB). The bad teams won about 1/3 of the games overall, won 3 of the 32 series, and tied four others. None of the bad teams won any season series that went beyond 10 games (so intradivisional), and only one tied such a series (BAL vs TB was 9-9). The longer the series, though, the less the chance the bad team wins it.