Four wheel drive not kicking in

Do you mean you have manual locking hubs in place of the vacuum locking units? Or are they the OEM vacuum hubs? A common issue with those hubs are vacuum leaks at the seals which cause them to stop working. This is a good video on checking them out (it is on a F-250 but the Ranger hubs work the same way, just smaller parts and your hubs are easier to replace but testing is the same).
https://www.youtube.com/watch?v=3zKYMm29Iig

As for fuses - Fuse 27 in the instrument panel fuse box powers the vacuum solenoid.

Check fuse three in the under hood fuse box. Beyond that you can do a simple check to see if the issue is the transfer case shifting or something mechanical. The transfer case shift motor is easy to unbolt and shift the T-Case into 4Hi and see if everything locks in.

Then have someone select the various 2/4 modes with the switch while you have the motor off.
Beyond that you need a scan tool to verify the switch operation and its control of the transfer case control module.

Just checking to see if you had any luck with your 4X4 issue.