If the rear springs on your truck are in good condition and can handle the weight involved, the most simple way of lowering the rear of the vehicle with the axle above the springs is to put a spacer block (called a lowering block) between the spring and the axle. You can add up to a 2" spacer without any issues with the spring function. More then 2" and you can run into a problem with the spring wrap up under acceleration. The spacer (or lowering blocks) are available in pretty much any inch increment, (1", 2", 3" & so on). You buy the lowering blocks that are the same width as the leaf springs you have. Adding the lower blocks may require longer U bolts holding the axle to the springs. The thickness of the lowering blocks is a direct amount of lowering, a 1" lowering block lowers the truck at the rear wheel 1" and a 2" lowering block lowers the truck at the wheel 2", you get to pick how much you want to lower the truck. The lowering blocks have been a hot rod go to thing for a very long time, they are simple, safe, and pretty cheap. My 48 Plymouth coupe that is on a Dakota chassis has had 2" lowering blocks for 12 years and nearly 100,000 miles.

The reason some have asked if the axle was above the springs or below the springs is because a 2 wheel drive Dakota has the axle above the springs but a 4x4 Dakota has the axle below the springs as the factory offering.