Only one wheel spinning on my truck?

The differential you have in your vehicle is not a limited slip style and will provide power to any wheel that will turn the easiest. Since the one tire was on grass there was less traction there and it was easier for that tire to spin than the one on the pavement. This is very normal for this type of differential and anyone with this type needs to be very careful to keep both rear tires on solid footing so you do not end up getting stuck. Some vehicles come with what is called a limited slip differential which does not allow for one tire to spin if the other tire can not spin as well. This gives you a great deal more ability to go off the paved road without the fear of becoming stuck. A limited slip differential can be installed in your vehicle in place of the one that is currently there, but the cost is usually somewhere between $1,000 and $2,000 for the complete job. Hope this helps.