I've just added an article on improving dynamic control (between limbs, and across an entire song). When I was first starting out, this really simple exercise helped me immensely to get more independence and better volume control between my arms and legs.
The TL;DR version is this: Hit 4 things all at the same time (e.g. snare, floor tom, hi hat pedal, bass drum). Slowly increase & decrease the volume of individual limbs one at a time. There are a lot of different variations on this - ranging from extremely basic all the way up to very complex.
If you're interested in the actual article, you can click here.
Feedback is appreciated, and let me know if this helps you!