Interesting question. You can look at this 2 ways: raw total difference or ratios. Ratio seems a bit more interesting, since guys can post monster stats in one category (while the other one isn't bad, there's a big split). But even for raw difference, Molina is awfully impressive even though his RBI count isn't that big. Right now, he's at -44. Howard last year was -45. Preston Wilson had a -47 in 2003. Andre Dawson was -47 in his MVP year. The only -50 guy I could find is Dave Kingman (-50 exactly in 1984).
Molina's current 0.397 run/rbi ratio might be a new record for a full-time player (I'm pretty sure Bill James compiled this list once, but I can't find/remember what it was). It's pretty hard to drive in runs and not score ... by driving in runs, you needs hits, and you have hits you must be on base a decent amount of time. And even the slowest runners should at least be able to move station-to-station when the following guys get hits. The man widely considered to be the slowest player of all-time, HoF catcher Ernie Lombardi, had some amazing ratios, though his plate appearances were rather low in some years. In seasons of 100+ games, his best (worst) ratios were: 0.373 (19/51) in 1943, 0.505 (43/85) in 1939, 0.550 (33/60) in 1941, and 0.563 (36/64) in 1935. So Lombardi's 1943 tops Molina for ratio, though his PT that year was limited (only 311 PAs). Dave Kingman might have the worst ratio for a 100+ rbi man: 0.576 (68/118) in 1984. Mo Vaughn's 1999 wasn't far behind: 0.583 (63/108).
Can anyone find anything worse than these guys (total and ratio)?
Differences the other way are obviously more common, especially for lead-off hitters. In just 2000, Luis Castillo posted an amazing 5.941 ratio (101/17). HoFer Richie Ashburn frequently excelled in this area: his best ratio was 4.3 (86/20) in 1959, and was 2.256 for his career. An earlier Phillie, Roy Thomas, posted a 5.063 (81/16) in 1906 and was 3.38 for his career. Don't know if anyone else cracked the 5.0 ratio for a full season. Biggest raw difference? Sliding Billy Hamilton had a +105 in his record-setting year. A number of "dead-ball era" guys had crazy splits like that ... I don't know if anyone topped +105 though.