When I did disney at 25ish weeks, I did ride the Safari (as well as some other rides that I am sure others on the board would shake their finger at me for) and I think when push comes to shove, we all have to make our own decisions. The big thing to remember that, heaven forbid, something negative should happen with your pregnancy are you going to be hunt for ways to blame yourself so that even if the causes for the negative outcome were not at all related to a ride at Disney, would you still beat your self up about going on the rides and assume the blame (A TOTALLY NORMAL RESPONSE TO THE SITUATION). Think about it. Also, during my trip, only one CM even noticed I was pregnant and it was en route to splash. She said "we don't recommend this ride for pregnant women." To which I replied, thank you for your concern and went on my favorite ride in the park.
As I've said elsewhere, like Pocahontas1, my OB supported my judgment in Disney and advised me to avoid anything with A LOT of force/gravity exerted (roller coasters, T of T, Mission Space). The bumps on the safari may be uncomfortable...but I didn't find them to be troublesome. Remember, Disney and your OB are giving advice partially motivated by sincere concern, but moreover motivated by the CYA attitude necessary to avoid litigation. But that they have the policies, suggest their have been negative outcomes and lawsuits...
Enough of my ramblings. Let your concious and your opinion of legitimate risk of harm be your guide.