Question:

In an ad, can my display URL be different from my destination URL?

Answer:

It depends on how you mean "different".

From the ad perspective, they can be completely different. But your resulting landing page needs to at least match your destination domain name.

The reason I say "resulting" is that the destination URL might redirect to another site. This happens if the destination is for tracking purposes, which in turn goes to the "real" landing page.

As for the landing page verses display URL, there is some room to "play" and to improve clickthrough rates, and quality score.

But I can't give those tricks away here.
I have those in my "Display URL Voodoo" guide, which is a bonus when you get "Google Slap Secrets"