Finally found the correct shocks for the IS. Monroe 555001.

Our IS was installed in June of 2017. We have towed "Brobdingnagian" ~ 40,000. I had noticed after the second year the trailer was bouncing more and more. Thought it was tire pressure or weight distribution or just bad roads. Well, turns out the shocks were shot. I pulled one off last week while repacking WB's. A three year old could compress them. Very easy to change out. Looking forward to a much smoother pull this year.