acdii

Measure the wheel wells empty, then drop the trailer on and measure again, then hook the bars and measure once more. You should be close to level front to rear when properly setup. If when you are fully hooked up and the rear wheel measurement rose, then the hitch is off. That will cause porpoising. If the front didn't change, that can cause problems as well.

Siskiyou

I have been adjusting mine, tightening the bars, loosening the bars, added LT tires which helped cornering a lot. You can tighten the bars too much and cause instability issues also. Here's what I did with my 02 Silverado which stock handled far worse than my current truck under load. Adjust hitch properly by measuring wheelwells and weighing first, added hellwig swaybar, changed to LT 6 ply rated tires and added one extra leaf to the spring pack. Handling problem was gone and worked well for the 15 years I owned it. The only difference I am doing with this one is I am going to add air bags instead of the extra leaf. I might add my trailer is about 6500lbs wet and loaded but only 24' overall length. I don't think you can ever get rid of porpousing entirely but you can get a very solid handling truck-trailer combination.

Nighthawk87

My new shocks helped a lot.

Timbrens were horrible. They got on speed bumps

SkiSmuggs

HD shocks like Bilstein 4600 or KYB will still help. I do think Timbrens may be a harsh stop, but take a look at Sumosprings which look to be kinder, softer Timbren.

ewdavis

Shocks and/or Timbrens/Roadmaster Active should help.

Be careful messing with WDH to stop porpoising. Travel trailers are heavy, and mine bounces alot. My WD platform is right on - truck suspension is just too soft and I have garbage Rancho shocks. I'm sure you guys would say the "GMC" is most of the problem

Also, most platforms (I have an Equalizer), adding a washer and going up one hole are the same for adding tension. READ YOUR HITCH INSTRUCTIONS. You should follow these for proper and SAFE hitch setup (don't guess at it).