Panic stop--why?

Roy pretty much nailed your brake problem. I'd lean toward the leak in the booster thought - an external leak doesn't quite fit the symptoms.

For the up-shift, how long since the transmission fluid was changed? That cold up-shift issue is usually my first clue that it's time to service the tranny (unless cold = what we have this morning: -8 F ).

The a/c question probably means that the cold temps above don't apply to you ;-) I'd be looking for a freeze up in the expansion valve. In South Texas, that was a common problem and the real cure was usually to evacuate the a/c system, let it sit with a really high vacuum for several hours to dry it out then re-charge. You may need a new accumulator - they have a dryer element in them to remove moisture and that may be saturated. Relatively cheap part if you already have the system evacuated.

Agreed. Take a can of carb cleaner and spray around the vac hoses with the engine running. if you find a leak the motor will stammer. Pay close attention to the booster hose where it flexes between the motor and the booster. It tends to crack. Also follow all the "hard lines" and check the boots as they crack as well. I replaced all my vac lines when the Jeep was about ten years old. Lots of cracks, splits, and it wasn't that difficult.

This Jeep is new enough that the tranny will store codes. If you have a DRB-II/III scantool (or access to a shop that will pull codes for you) that might help in isolating the issue. But do check fluid level. Cold (real cold) trans fluid will cause problems. It does in my ZJ.

Power for the clutch runs through two pressures senders. A low side located on the accumulator (big can) and high side located on the compressor. The low side sensor could be failing or you might be low. Alternatively if someone stuffed too much refrigerant in there you might be too high. Hard to tell from behind a keyboard. :)

Next time it cuts out, disconnect the plug from the accumulator sensor and short the connector's pins out. If the AC clutch kicks back on then it's probably the sensor. That part can be replaced without having to mess with the refrigerant. You just screw it out (fairly quickly) as there is a tiny valve (like a tire valve) under it. Then install the new unit. use a little soapy water to check for leaks and don't overtighten it or you can strip out the aluminum mount.

Oh.. how was the weather? wet/rainy/icy? Sometimes drum brakes can build up ice inside and cause problems like that. A real bad case I saw once was ice buildup on the caliper! Also check the fluid level and examine the lines to see if one is blowing a bubble.