I dont know much about sled pulling, but sounds like your tires are digging a little hole then pulling themselves out when they find something a little more compact. I've always heard that you want narrower tires for snow and mud so it digs into the solid stuff below instead of riding on the loose stuff up top. Sounds kind of like you're headed in my trucks direction. Shackle flip and a nice set of shocks. I can say that my truck rides a ton nicer with the shackle flip compared to before. Jumping on the bumper made little flex in the rear before. Now you can easily move it a couple inches or so. Much much smoother in the rear. Coilovers and leaf springs are pretty similar to each other when comparing same spring rates. The advantage for coilovers is the ease of adjusting the ride height as well as well as being able two have two or even three different spring rates based upon the travel position.