Here's a breakdown of likely causes:
* Transfer Case Linkage: The linkage connecting the transfer case shifter to the transfer case itself might be broken, bent, disconnected, or severely out of adjustment. When in neutral, this linkage may be preventing the transmission from selecting gears properly. This is the most probable cause given your description.
* Transfer Case Actuator (if electronically controlled): Some transfer cases, especially in later models, use an electric actuator. A faulty actuator, wiring issue, or low voltage could prevent the case from engaging fully into neutral, interfering with gear selection. (Less likely in a '93 but possible depending on the specific model and any modifications).
* Transmission Shift Linkage: While less likely given you can select gears in 2WD, it's possible there's an issue with the linkage between the transmission shifter and the transmission itself. A bent or broken component, or a problem with the shifter's mounting could affect gear selection, potentially in a way that is exacerbated when the transfer case is in neutral.
* Internal Transmission Problem (unlikely): While less probable given your description, a severely damaged internal transmission component (like a broken shift fork) *could* theoretically manifest this way. However, the ability to shift in 2WD makes this less likely.
Troubleshooting Steps:
1. Inspect the Transfer Case Linkage: Carefully examine the linkage connecting the transfer case shifter to the transfer case itself. Look for broken or bent parts, loose connections, or anything out of alignment. This often involves getting under the truck.
2. Check Transfer Case Operation (with the truck off): Try manually shifting the transfer case into 4H, 4L, and Neutral. Does it move freely? Is there any binding or resistance?
3. Test 2WD Shifting Thoroughly: Ensure you can consistently select all gears in 2WD without any problems. This helps confirm the issue isn't primarily a transmission problem.
4. Check for Electrical Issues (if applicable): If your truck has an electronically controlled transfer case, check for any electrical problems related to the actuator, its wiring, or fuses.
Important Note: If you're not comfortable working on your vehicle's drivetrain, it's best to take it to a qualified mechanic for diagnosis and repair. Improperly diagnosing and repairing drivetrain components can lead to further damage.
