I wouldn't like to suggest wikipedia as the definitive medical text, but...
http://en.wikipedia.orgwiki/Diclofenac#Contraindications
Which is an example of an old-fashioned NSAID, the more modern like the COX2 inhibitor
Celecoxib (celebrex) is less definitive about Crohn's, but does say:
In theory the COX-2 selectivity should result in a significantly lower incidence of gastrointestinal ulceration than traditional NSAIDs. The main body of evidence touted to support this theory were the preliminary (6 month) results of the Celecoxib Long-term Arthritis Safety Study (CLASS) as published in 2000, which demonstrated a significant reduction in the combination of symptomatic ulcers plus ulcer complications in those taking celecoxib versus ibuprofen or diclofenac, provided they were not on aspirin (Silverstein et al., 2000). However, this was not significant at 12 months (full study length).
So, as ever, your GI should be the one to give you advice.
However, be prepared to ask if you really should be on the any of the NSAIDS/COX2 inhibitors if you have Crohn's or UC because not all medical staff are aware: I was very nearly given IV diclofenac for post-op pain despite the conversation with anesthetist beforehand where I'd told him of a history of suspected stomach ulcer, diagnosed Crohn's, and asthma, and requested opiate pain relief instead. Which reminds me; the method of delivery is immaterial. If you shouldn't have NSAIDS you should not have them as a topical gel, IV, or suppository. It doesn't have to hit your stomach to do its damage!
Another thing to bear in mind is the interaction of NSAIDs with methotrexate which isn't normally a problem with the dosages used for Crohn's or arthritis, but just gives me another reason to be wary (of both!).