Did you try airing up the 37's to see if that would help with the "squishy"? I added 37's but haven't towed my boat with them yet. I only tow a couple of times a year so not too worried about it. I might just air up for my short trip from storage to the lake. I used to air them up when I towed it longer distances and noticed the difference in the ride with more psi.I'll add that I tow monthly with OEM C rated 35's. I swapped to a set of takeoff OEM C rated 37's for 2 trips and put my 35's back on. Though I'm well within the weight rating for both tires, the C rated 37's added a squishy, lack of control feeling while towing that I didn't like at all. If I move to 37, it would likely be a D rated tire as well. Did you lose any ride quality with the switch to D?