Need help my heater works great .... but only on the window defrost when I switch it to the floor or bi-level it does nothing only blows on the top any ideas what it can be

 

Those vents are controlled using vacuum actuators. When they fail to work it can be because:

1. The selector on the dash is mechanically broken or disconnected.
2. The actuators have failed, but the vacuum supply is good.
3. The actuators are good, but the vacuum supply is bad.

The vacuum is supplied by the motor via vacuum air lines, and reserve vacuum supply is held in the plastic vacuum reservoir that is tucked inside the passenger side end of the front bumper. If that component is missing or leaking, or if the vacuum lines from the motor are disconnected, cracked, leaking, etc, it can cause these problem.

If the vacuum source and lines are tested to be in good shape, then the troubleshooting should move towards the dash and the actuators and selector.

There are many searches that can be done for this. Things like "Jeep XJ defrost only" could get you started. However, if you search on something as general as "Jeep xj no heat" you are bound to pick up a bunch of heater core and thermostat related posts that won't be helpful.

Let us know about your progress. Good luck.