Heat coming from one side only?!?

Ok, I have asked this on a couple of other forums, and I can't seem to find anyone who has had a similar problem, or who knows what going wrong. When I turn on my heater to either vent or defrost some hot air will come out of the selected vents, but cold air will blow in through the passenger foot vent (doesn't matter what is selected). If the heater is off, and I am driving fast enough (i.e., freeway speeds) I can feel a draft coming in from the passenger side foot vents. I know I have some vacuum problems because the hot air coming out of the vents fluctuates. Could this also cause the weird issue I'm having?

Always post your Cherokee model year and any other engine, transmission, transfer case, axle ID info that applies. There are numerous variations in Cherokees over the years and even simple answers will vary greatly based on your Cherokees installed equipment or electronics.

Yeah, sorry, forgot to list that.

It's an 87 Laredo, 4.0L and it does have AC (if that could affect it).

When you have a known problem that is at least partly related to other issues, you should fix the known problem first as it may mitigate or alleviate the other issues.

In other words, yes, fix the vacuum system.