I just noticed the truck, I used to have a 90’s 2500 Silverado w/bonus cab and 8’ bed and ran a western pro-plow on it in winter. It always felt better with ballast in the back (plow or not). I seem to recall running around 500 lbs in it so that’s consistent with your experience as well. It felt like a lumber wagon when empty.
When I bought the truck, it had Goodyears on it and didn’t need new tires due to wear but they stunk for traction. I ended up with Goodrich Long Trails and it was night and day. I could even plow in 2WD with those on it.