Jedi Dark Knight said:

the whole thing is to represent the metaphor: "needle in a haystack"; Han is gone, and they don't know where, and their haystack is HUGE, so the unsurmountable task is presented visually like that.

I don't remember, did they ever specifically mention Jabba toward the end of ESB?

I know Han said Jabba put a price on his head, and why not collect a double bounty, from the Empire and then from Jabba? Tatooine is the logical place to start looking, and it was their rendezvous point, after all. It seems they knew he would be taken there; I just don't remember if Vader or Boba ever mentioned Jabba...