I have a 90 ranger 2.9 4x4 originally auto but previous owner 5 speed swapped it. Has the push button 4x4. It was working fine from 2wd to 4hi but 4lo never went in it just sat there and blinked. I was driving in the snow last night, was going from 2wd to 4hi and back periodically as needed and was working just fine. I parked it at a buddys house for a couple hours as we were shooting the ****. When u came back out to start it it was in 4wd and the low range light was blinking. I tried getting it back to 2wd but the buttons did absolutely nothing. Its been stuck in 4wd with the low range light blinking most of the day. Im not sure if its a vacuum issue or if maybe the actuator is bad or maybe even the button assembly is faulty. Any input would be greatly appreciated. THANK YOU!

 

Welcome to the forum

No vacuum involve in 1990 4WD system, that was only on the 1998-2000 Rangers

You have an electric shift motor on the transfer case, it is operated by a shift control module, which gets its input from the dash buttons

Usual failure is the shift motor, the "moving parts" of the system.
Try tapping on the motor with a hammer or wrench, that can get it working again, not always but it is FREE to try it, lol.

Flashing 4WD light means shift motor is not moving when controller "tells" it to.

Good read here, with pictures about shift motor: How to: Rebuild 4X4 Shift Motor | Ford Explorer and Ford Ranger Forums - Serious Explorations

Shift module troubleshooting here: Ford Ranger 4WD Control Module

You could have just blown the fuse for the shift motor, it would be in the engine bay fuse box

 

Sometime you can put it in 2WD, back up and pump the brakes, hit the gas, pump the brakes OR try the same thing going forward.

Just something that helped me, can't say it will help you for sure but give it a try.

ALWAYS take your truck out of 4WD before shutting down !