a few things to check:
1 - Be sure that the dampening on the struts are set evenly. Adjusting the dampening up or down may dial in the slight sway as sometime hwy conditions can build up an oscillation.
2 - Check tire pressure. If it is low, with the larger size you are running, the pliable side wall can roll back & forth at hwy speeds. We normally set tire pressures at the higher side of their range to stiffen the side wall. Cargo trailers are normally equipped with ST designated tires that run at higher pressures for this reason.
3 - Check for any bearing play in the hubs. Simply grab the top of the tire with the trailer on the ground and push/pull side to side. If you hear/feel a slight clunk, the bearing pre-load needs to be reset.
4 - After verifying items 1-2-3, do a quick check of toe-in adjustment with matched set of tape measures and straight edges along the sides of the tires. We set toe-in 3/16 - 1/4" at the shop. Since we had a set of smaller tires & wheels on the trailer for delivery and the new tires & rims were mounted at your place, there may have been a shift.
When in doubt, give us a shout at the shop. We'll take care of you.
We had an unusual case once that a slight sway condition was solved running some MTRs by switching out the rims from 10" wide to 8". The problem was sidewall stiffness related.
