Jimmy wont lock in four wheel drive.

The reason the light is flashing constantly is because the vehicle is sensing that four wheel drive(4WD) is not working. These 4WD systems use vacuum from the engine to engage/disengage to front axle. The vehicle probably has no vacuum going to the front axle, which would not allow the 4WD shift to complete, which would then leave the light on the dash flashing.
First thing to check is that the vacuum actuator on the firewall is working as it should.

Which do you have ?
Transfer Case - NVG 236 (Two Speed Automatic)
Transfer Case - NVG 233-NP1 (Two Speed Selectable)
The NV236 transfer case is an automatic transfer case with three drive ranges. Shifting from rear wheel drive to four wheel drive is done automatically when the transfer case shift control module receives wheel rotating slip information from the speed sensors. The transfer case shift control module then engages the transfer case motor/encoder to position the transfer case from rear to four wheel drive. When the transfer case shift control module receives information that the wheel rotation is the same on both axles, the transfer case shift control module sends position information to the motor/encoder to put the transfer case back into rear wheel drive. The three drive ranges are 2HI, 4HI, and 4LO. The 2HI position is for normal driving with the rear wheels pushing the vehicle. The 4HI, position is for driving through sand, snow, mud, gravel, or heavy rain at normal or slightly below normal speeds. The 4HI engages the front axle to allow for four wheel drive. The front axle is pulling the vehicle, and the rear axle is pushing the vehicle. The 4LO range is used for off road driving, heavy snow, deep mud, or shallow fording. The 4LO is similar to the 4HI except that the gearing in the transfer case is set for low speed and high torque. The NV236 is manufactured at Syracuse New York by New Venture Gear a division of New Process Gear for General Motors.
Is the switch light on or flashing ? If you have automatic 4x4 there are several DTC'S that can set >
DTC B2725 ATC Mode Switch Circuit Malfunction DTC C0300 Rear Speed Sensor Malfunction DTC C0305 Front Speed Sensor Malfunction DTC C0308 Motor A/B Circuit Low DTC C0309 Motor A/B Circuit High DTC C0310 Motor A/B Circuit Open DTC C0315 Motor Ground Circuit Open DTC C0323 T-Case Lock Circuit Low DTC C0324 T-Case Lock Circuit High DTC C0327 Encoder Circuit Malfunction DTC C0362 4LO Discrete Output Circuit High DTC C0367 Front Axle Control Circuit High DTC C0374 General System Malfunction DTC C0376 Front/Rear Shaft Speed Mismatch DTC C0387 Unable to Perform Shift DTC C0550 ECU Malfunction DTC C0611 VIN Information Error

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To shift into 4lo you must come to a complete stop, put the transmission in neutral, and step on the brake pedal, then push the 4lo button. To shift out of 4lo, do the same thing. If it's not going into 4hi, check every vacuum line you can find. Everything is vacuum operated. If your vacuum lines are old and crusty or mushy, they may collapse when vacuum is applied. There is one vacuum hose that is hidden under the battery tray that almost always comes loose, and it is the one that locks the front axle. You will need to remove the battery and tray to get to it. If the hose is dried out or stiff, replace it with a new one that is flexible enough to make the sharp bend required to hook up to the actuator. Also check the actuators under the vehicle at the transfer case and front axle.

ok what i would try is put the truck in park then try to **** into your 4wheel drive high or low, other than that with the computer thats on board you may just have a technical issue that can be solved at the dealership

You should be able to put in 4x4 while in drive less than 35mph. Mine had a switch to turn to engage or dis-engage.

Change shift solenoid

ithink it might be just a bad senser

IF YOUR SHIFTING INTO 4HI or auto mode can do at any time under 35mph. going into 4lo you need to be in neutral stopped usually goes smoother if foot is off brake and it can roll a little same with coming back out into 2wd