I was at AK when I was three months pregnant and I didn't go on the safari because of the warnings posted. A year later, I rode it and thought there was nothing bad about it at all. But last March I rode it again, and it was very bumpy and I would not have liked it if I was pregnant. Some bumps were very jarring - I actually popped up off the seat.
The problem with making a recommendation is that the weather and other conditions affect the ride each day. Some days it might be bumpier than others.
At five months pregnant, I would suggest your wife skip the ride. If she's bummed about missing the animals, take her to AKL for dinner or lunch, and walk out to the viewing areas and sit in the rocking chairs on one of the open balconies, where you can also watch the animals roaming around.