Hey,
i had the same problem, like well strong abs but hardly any definition. I recently ran the London Marathon, which required a lot of aerobic training, like running and, a lot less anaerobic training, like weights, but this gave me alot of tone and definition.
So in answer to your question, you might want to start running for short to medium distances(5k-10k) as this will get rid of alot of fat, but at the consequence of losing a bit of bulk, where the fat has been taken away from your arms and shoulders.
Hope this was some help to you.
Stev0