Air or heat only comes out defroster. vacuum lines are fine. cruise control works. is it a switch or the vent actuator?

The mode door is what shifts the airflow. The door is controlled by a vacuum actuator. So you've either got a bad actuator, leaking vacuum line or a bad vacuum switch.

check under the hood and locate your vacuum lines going in and out of the fire wall, a number of times, vacuum lines can crack and sometimes cracked lines can seal, or leak.

if all your air or heat comes out of your defrost than the problem is more than likely your switch or a vacuum hose to the switch. Car's default to defrost when there is a problem. I can't recall off the top of my head but I do believe that vacuum controls the blend doors on the 98 Ranger. I can't quite remember but I think the white one is the main vacuum. Either way. pull out your heater switches, disconnect the rubbery plug with all the vacuum hoses, start the engine....1 of those should have constant vacuum, if not...chase it.....there will be 1 that goes through the firewall to the engine, the rest go to the blend doors.

Check taht the dash controls are moving to the selected option.Check for broken cracked vacuum lines to actuators for vents

There are 2 vacuum lines that control your hvac system, one of them is either disconnected or dry-rotted and needs replaced. I was able to fix it by purchasing a 3-way splitter for a vacuum line, bought some rubber vacuum line and spliced into the cruise-control vacuum line. Worked like a charm. If it's not that, you could have a vacuum leak anywhere alone the line from the intake manifold to the evap canister

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

The problem is probably a faulty diverter door actuator or vacuum problem at the HVAC plenum under the dash. This is a common problem - see YouTube for help diagnosing this:
https://www.youtube.com/watch?v=Nlt9A1sp-4U

There is a vacuum leak under the hood somewhere. Check the vacuum at the speed control servo. If there is no vacuum on the line with the engine running, follow the line to find the leak. The vacuum reservoir is mounted under the battery tray. I would look in that area after confirming no vacuum at the servo.

prbly have a leak in your vacume lines, have to pull the heater controlls and make sure every things conected and not leakin if its good youll have to trace your vacume lines farther

there are a few things that can inhibit the cruise control on a jeep, and the more prevalent one is a vacuum leak in that circuit (hose) that runs off the intake manifold (rearmost one with the check valve in it), which then jogs over to the passenger side of the vehicle, and takes a dive down under the battery tray to the vacuum reservoir. if you are creative about making a plug after pulling the hose off the intake manifold, and then disconnect the tiny hose from the cruise control actuator, also on the passenger side of the engine compartment, and then pull vacuum either with your mouth or a small vacuum pump, then you've proven that this is not the reason the system is not working. Also, if your valve cover rearmost vacuum line to the intake manifold is 'leaky', it will cause this as well as other issues, such as defroster or air conditioning not to blow thru the air vents into the crew cabin, but only onto the windshield defroster vents. if this turns out not to be the problem, and the hose and valve cover fitting are in good shape (the center section of this is plastic and brittle and subject to cracking and breaking, by the way!), it's a good time to start to suspect the brake pedal switch. this switch has contacts that do more than just allow the brake lights to come on, there are cruise control 'inhibit' contacts in this switch as well. and yes, these do tend to go bad. in any case, if the instrument cluster blue light never comes on, it suggests that the system has more than vacuum problems and this brake pedal switch is most likely the cause if there is no other known electrical problem, such as an open circuit for the cruise control. I wish I could offer more, but the liklihood of the actuator being the root cause of this failure, is pretty slim, and there is not much else for you to play with, though there is probably an Engine Control Module possibility here, as a group of five or six Cruise Control wires actually go into that box. In any case, do the vacuum hose and vacuum checks first, then start to think about the 30 dollar switch on the brake pedal.