Well, the landspeeder we actually had photo set reference of, we knew it was going to be in the movie, so I made one.
The sandcrawler was completely built by one of our level designers on a whim, just a side-quest for something to do. I think you get some flash grenades if you actually rescue the R2 unit.
No, there wasn't any communication as to the sandcrawler or the landspeeder. We knew there were Jawas, so we naturally assumed there would be a sandcrawler too.