A/C not working properly

Being an 88 the first item to check would be the vacuum lines that operate the HVAC doors. There is a single line that comes off of the intake manifold then to the vacuum reservoir, it comes out of the reservoir to the control head. Inside the control head are a bunch of ports that direct the vacuum to the different vacuum motors that move the doors. If any of the lines fails, which is common as they are rubber tubing that doesn't age gracefully, you end up with erratic operation. Your description sounds like the issue might be under the hood as the vacuum line is easy to knock loose if you breath on it wrong. The underhood lines commonly get rotted and when you touch them they feel like goo and fall apart. The lines under the dash can be a pain to work on but if you pull the trim and then the controller you can get to the block on the back that the multi-connector lines run to. From that you can use a hand vacuum pump to test each of the ports to find the problem.

Hmm, two different years listed. Doesn't make a lot of difference though, the 98 still uses the same vacuum controls for its heater. Jeep didn't change much over those years the Grand used a lot of the same parts as the lower Cherokee and shared them up until 1999 when the new version came out. These are the 98 vacuum lines; the connector is a different shape, but it still uses vacuum for all the doors. One change though, they relocated the vacuum reservoir to the heater box behind the dash instead of in the fender.

The small ones behind the dash are either 1/8" inch nylon with rubber couplers or 3/16" rubber. I generally have rolls of the stuff on the rack and just match it up as needed but those are the common control sizes. Another area that fails is the internal seal in the control head itself. They wear and leak, usually though you would know if that was the case as the system will work in one mode then not in another, or it will work fine at idle and then slowly fail as the rpms go up. Not working in any mode is commonly the main vacuum line or the reservoir.

That year is around the time they started switching to the nylon stuff with fittings. If it doesn't have it, I would convert it, the plastic seems to hold up better than all rubber. Most parts stores will have a rack of the various fittings and line. Just drive it there and match things up.