I just picked my first xj, shes a 98 xj sport, I have a question about the ac. When I turn on the ac to the dash vents it I hear it blowing but only comes out of the very top vents on the driver and passenger sides,, but not any of the main vents on the dash, when I switch to the floor vents they blow as well as the defrosters, my fuses are good, and from what I can tell no vacuum lines are cracked. Any advice will be much appreciated.

 

A little late to assist, but still.......

Anyway, I think you missed a vacuum leak somewhere. The vent selector works off a vacuum so you need to start at the intake manifold and follow the vacuum line around where it "Ts" where the heater lines enter the firewall. One leg of the "T" will go through the firewall and connect to the vacuum system in the dash. The other leg of the T goes around to the cruise servo if you have cruise and then it goes to the vacuum reservoir that is behind the right front bumper. It will look like a black plastic softball only slightly larger like a small cabbage. Check those out and report back.
Good Luck!

 

I followed the vacuum lines to the vacuum reservoir and pulled the hose off to inspect the reservoir for cracks and when I pulled the hose off it made a hissing sound like it was pulling air in, so I would assume it's able to keep vacuum?

 

There's a small diameter plastic vacuum capillary tube that enters the firewall on the upper passenger side. That tube provides vacuum switching for the valve dash pots in the dash. Because the tube is of such a small diameter it will not cause a significant vacuum loss across the entire vacuum system, but it will disable your HVAC control in the dash. Look for that line and inspect it. If it's cracked you'll need to get a small section of black vacuum hose from your local auto parts store to make the repair. Be careful. If cracked it's because it's age-deteriorated and will readily crack again if roughly handled. You'll want to break it at the crack and bridge the two ends with the rubber vacuum tubing.