1990 GMC Sierra 4x4 1500

Your best bet , take it to a qualified repair shop , before you replace more part's that arn't needed . Did you check to see if any DTC'S - diagnostic trouble code's are stored in the transfercase control module . Which transfer case does it have ? How many buttons on the instrument panel 4X4 switch ?
The NVG 236/246 transfer case features a 4 button shift control switch located on the instrument panel.
Do you know what a DTC is ?
DTC C0367
The front axle control circuit consists of an electric motor actuator which engages and disengages the front axle. The front axle actuator's motor consists of a Permanent Magnetic (PM) motor, a worm gear controlled plunger, a front axle switch and an electronic control circuit all within the actuator assembly.
The front axle actuator has four circuits connected to it, a front axle feed circuit, front axle ground circuit, front axle control circuit, and a front axle switch circuit.
Whenever a shift to AUTO 4WD, 4HI, or 4LO is requested, the automatic transfer case control module engages the front axle by grounding the front axle control circuit through a current limiting driver.
This DTC detects a shorted actuator, or a short to 12 V on the Front Axle Control Circuit.

Your vehicle can set a number of them .
DTC B2725
DTC C0300
DTC C0305
DTC C0308
DTC C0309
DTC C0310
DTC C0315
DTC C0323
DTC C0324
DTC C0327
DTC C0362
DTC C0367
DTC C0374
DTC C0376
DTC C0387
DTC C0550
DTC C0611
Without a professional scan tool an factory service repair info. you really shouldn't be trying anything , take it to a qualified repair shop.
The NVG 263 transfer case features a three button shift control switch located on the instrument panel.

New Venture Gear 263 Selectable Two Speed Transfer Case
The NVG 263 transfer case features a three button shift control switch located on the instrument panel. When the vehicle has the ignition key in the RUN position, the transfer case shift control module starts monitoring the transfer case shift control switch to determine if the driver desires a new mode/gear position. At a single press of the transfer case shift control switch, the lamp of the new desired position will begin flashing to inform the driver that the transfer case shift control module has received the request for a new mode/gear position. The lamp will continue to flash until all shifting criteria has been met and the new mode/gear position has been reached (or has been engaged). Once the new mode/gear position is fully active, the switch indicator lamp for the new position will remain ON constantly.

The actuator will not be on the front differential or drive line, it will be on the transfer case, which is mounted behind the transmission. Follow the front drive shaft back from the front differential, and the component at the rear of the driveshaft is the transfer case. That is where four wheel drive is engaged/disengaged.
I'm not familiar with your vehicle, but if you have a separate lever to engage 4wd, there will not be a fuse for the actuator - it is a manual system. If the 4wd is selected electrically, check that all wiring and connectors on the transfer case are in good condition.

I would think so..

Sounds like you need a front axle actuator. This actuator bolts to the side of the front differential housing. It is electrically engaged. You can purchase them online starting at about $40

• Shifting from 2HI to 4HI Press and release the 4HI button. This can be done at any speed, and the front axle will lock automatically.

• Shifting from 4HI to 2HI Press and release the 2HI button. This can be done at any speed, and the front axle will unlock automatically.

• To shift from 2HI or 4HI to 4LO, the vehicle must be stopped or moving less than 3 mph with the transmission in NEUTRAL

• Hope this helps.

older GM's have an electric motor on the transfer case to engage the front wheels. Since it is under the truck they do corrode. Look there first. Easy to fix if so.

If yours is electric shift, you will need a new shift solenoid installed in the tranny.

More than likely ..electric motor on transfer case.....(this is what activates your front drive shaft in conjunction with another actuator to engage the front axle for 4x4 operation)....... malfunction

actually that first soultion is wrong it is correct on smaller pickups but on the full size pick ups they use a differnet sytem
there a electric actuator on front axle
there be two elctrical plugs on front axle u want the bigger of the two other one is just to turn the 4 wheel drive light on
un plug actuator then remove actuator after it removed plug it back in but leave it out of axle turn key on and shift down to 4 wheel drive the actuator should move out if it dont that your problem if i does move this isnt your problem it either the tranfer case or in front axle

Check front axle actuator see if it came unpluged