My 96 XJ sport blows luke warm air through the heater and changes from vent to defrost when going up hill, how do I fix this?

I'm not the under-dash guy, but it sounds like it's possibly the blend door. Do a search here, there are numerous threads about it.

Post back and let us know if that solves the issue.

Lukewarm - check the hoses going to the heater core, both should be hot (be carefull). If not, flush the core and the heater valve. If so, check the blend door as Fred suggest.

Switching vents - If you're going up a big hill with a heavy load, this may be normal. Vents are vacuum operated. Low vacuum levels can force the vents to defrost (the default). Otherwise, you likely have a vacuum leak in the HVAC system somewhere. There are plenty of posts with diagrams already here. Try searching a few out.