Have you tried caffeine-free Pepsi? I know, caffeine free drinks usually taste horrible in comparison, but caffeine really messes me up (and some others here). I don't even dare drink a whole can of cola (I don't drink any at all, actually).
I used to be a cola junkie. When I was a teen, I could down a 3 liter bottle of Mountain Dew over the course of a Saturday--definitely a 2-liter bottle a day during the summer. I drank so much I made myself immune to caffeine. In college I could have two iced expresso coffees and then go home and go to bed and sleep well. Then, right after I got out of college, I made the connection between getting sick (bad gall bladder) and drinking colas, so I gave them up. Luckily I wasn't dependent on coffee too; I just had dessert coffee every great once in a while. And I got my grandmother and parents to switch over to decaf iced tea for me too. When I stopped having diarrhea everyday, boy, I suddenly didn't miss those colas at all.
After I got my gall bladder out, I tried drinking colas again, but sometimes they made me sick and sometimes they didn't. I went on and off them for about 2 years before I finally decided, you know what, I'm tired of never knowing if this coke is going to be the one to make me sick, so I gave up caffeine all together again. The most I can tolerate is what's in green tea (not much at all).
I wouldn't miss cokes (or have even taken them up again) except my fiance drinks them some, and seeing them in the fridge makes me want them. But I have found a good alternative: Stewart's Black Cherry Fountain Cola. I don't know if they do a regular cola or just the specialty drinks, but cherry coke is what I miss the most, so I have taken to these. Man, are they good! No caffeine and very little carbonation, so they don't bother me at all. You might try giving up the cola-a-day (or at least switch to non-caffeine, although the high carbonation causes a lot of people gas cramps) and see if that makes your diarrhea any better. Like I said, as soon as I made that connection, I gave the colas up. I hated having to work and go to the bathroom all the time a lot more than I loved colas.