AC out of Defrosters

On high, my AC switches to the defroster but only when I am driving on the highway. From other threads, I think I am looking for a leaky vacuum line that comes out of the firewall by the heater hoses and that leads to a vacuum canister in the front bumper. My only problem is that my front bumper was removed by the previous owner. What does the vacuum canister look like? I don't see a white vacuum line anywhere. My Cherokee is a 1997. Any good ways of isolating this?

Older Jeeps looks like a football, newer Jeeps a ball. Yours should be the ball. As to where it was moved, if it was retained, anyones guess. People move them to all sorts of different places under the hood. I would start looking at the passenger side front corner as that is where the stock location is.

There should be a vacuum diagram somewhere under the hood - you should be able to use that to find/trace the vac lines until you either find the reservoir or discover you don't have one any more. There aren't many vac lines on a 4.0, so it shouldn't take long...