I think what you are feeling are worm castings that have dried and hardened at the surface of your lawn.
Earthworms (large ones – night crawlers) burrow up to the surface at night to feed on the decaying grass layer (thatch), mate, and absorb water from dew that has condensed on the grass. When they come to the surface, they leave a small mound of their castings (or “poop” for lack of a better word). These castings look like piles of small mud balls. When they dry, they harden into hard mounds, making it feel like you are walking over golf balls. If you part the grass in an area where you feel one of these golf ball mounds and remove some of the castings, you should be able to see a small hole where the worm has surfaced.
You must have a pretty healthy population of earthworms in your soil to have so many of these mounds. These worms are good to have in your lawn because the small tunnels they create as they move through the soil aerate the soil allowing good penetration of air and water. Their castings also add organic matter to the soil. This is great for your turf. There is nothing that you can really do to prevent this.
Do not roll your lawn because this will just compact the soil and make it hard for air and water to penetrate the soil. This is not good for your grass and it will also not do anything to eliminate or smooth out the dried worm castings.
Dethatching (if you have a heavy buildup of thatch – greater than ½”) can help to break up the dried worm castings and will also be good for your turf. This is best done in the early spring before the grass begins to regrow or in the fall.
Fertilize the lawn with a good quality organic fertilizer like Espoma Organic Lawn Food in the spring and fall according to the label directions and water your lawn slowly and deeply if conditions become dry.
I guess the bottom line is to embrace the fact that you have good healthy soil with lots of earthworms helping to keep it that way and providing their own organic “fertilizer” by leaving their castings behind them as they move through the soil!
I hope this helps to clear up the mystery of your bumpy lawn!