Well I bought this Ranger a few weeks ago and the air won't blow through the vents on the front of the dash. It acts like it's stuck in defrost. Anyway after a bit of messing around I found out that it will start to work if you switch the truck to 4x4. It works for a few minutes or until you turn then it shuts off but it will engage again after you turn it back to 2wd. Any ideas?

btw the truck doesn't have A/C at all it's an XLT.

 

Welcome to the forum

Air vents in the cab are run by Vacuum, default/no vacuum position, is Defrost vent
There is a Vacuum reservoir in the engine bay, black plastic ball
When engine is running intake vacuum is stored in this ball, same as it is stored in the power brake booster.
When you accelerate vacuum in the intake drops to 0, so without the vacuum reservoir air vents would switch to defrost(and power brakes wouldn't work, lol), until you stopped accelerating and vacuum returned

When air vents don't change at all with engine running, 95% of the time it the vacuum hose from Firewall to reservoir.
If you look on the passenger side firewall down in the heater fan area you will see TWO vacuum hoses, one black and one grey, they are hard plastic so can look like wires.
The grey one will run up to heater hose valve
The black one is the one you want to follow/trace, it will run to the vacuum reservoir, and it runs near exhaust pipes so..........can often melt or crack

You may find a "T" on the line, that is for the 4WD, the front hubs are Pulse Vacuum Hubs(PVH), so need a stable vacuum reserve
This "T" may be why you had vents for a short time, PVH system may have a leak in 2WD but not in 4WD

Once you find the Reservoir ball it will have a second vacuum hose, that hose runs to the intake manifold, follow it up and make sure it is good all the way.