How about slowing down? If your talking anything like the frost heaves we get around here then nothing is going to help unless you slow down.
I would think for the size of trailer you're towing the Anderson would be the worst hitch to use. The dealer sold me on one when I got my 28 foot toy hauler which runs 7200 lbs dry and around 10000 lbs loaded and I can tell that it just barely is enough hitch. I have thought about changing to a bar type hitch a couple times but I do like the lightweight Anderson and in the end I guess I just don't care as I have towed through some heavy Wy winds without any problems but then again I have towed 1000's of miles across the West with no weight distribution hitch on many different trailers and never had an issue.
I would think that a 2500 long bed pickup would handle your 8000 lbs trailer just fine without any WD. How fast are you towing? I'm not busting your chops on speed as I tow 70+ down the interstate and 65 on secondary roads when conditions permit, just don't tell the speed and tire police around here or this thread will do to hell.
Is the porposing normal for the road conditions and you don't realize it? Are you new to trailer towing? Hate to see a guy throw a bunch of cash at a problem that isn't really a problem and isn't going to be fixed. You have to expect some porposing with a truck and trailer as well as some pushing and pulling due to wind conditions.
Last time we came back from your favorite camping spot I was towing along on a secondary road at 65 when I noticed I was getting to the rough section of the road. Behind me was a guy pulling a boat, he probably was wondering why I went from 65 to 50 as he passed me. Shortly after passing he realized why I slowed down when he hit the first of 3 frost heaves and his 20 foot fiberglass boat and trailer came off the ground as he pounded through the heaves. I went through the heaves at 40 ish and my outfit proposed fairly well.
