To find the correct size, you need to:
1. Visually inspect the nut itself: This is the most reliable method. Use a socket wrench set and try different sizes until you find one that fits. Note the size and make sure it's a proper, snug fit.
2. Consult a repair manual: A repair manual specific to your 2000 Dodge Dakota 4x4 will list the correct sizes for your axle. Chilton, Haynes, and factory service manuals are good options.
3. Check online forums: Dodge Dakota forums may have discussions where owners with similar trucks have mentioned their axle nut sizes. However, this method is less reliable than the first two.
Warning: Working on your vehicle's axle requires significant mechanical knowledge. Incorrectly tightening or loosening these nuts can lead to serious damage or injury. If you're not comfortable performing this task yourself, take your truck to a qualified mechanic.