New tire issues

[Shack]

2007 Expy limited. Just purchased new Michelin ltx m/s2 20". The ride is quiet, great in rain. (75 mph in down pour, felt no different than dry pavement). However on wash board or patched roads the tires feel wobbly or jiggly. I know they are a softer side wall for better ride oh highway. Running factory settings 35psi w/nitrogen. would upping the pressue to 38 or even 40 help this. I frequently have a full load of people. Very little of road.