When you say hips, could you specify which muscles you find overly tight when the back leg locks out? I ask ,because its difficult to communicate these things in the written word, and I find it's an easier basis for discussion when we can use anatomical terms. That way, we all can agree what we mean by things like "hips," "back" etc.
i used to straighten out the leg too until i was told that i created tension around the area between my hips joints, the creases in front of my crotch. also, the opposite muscles behind my buttock also tensed up. essentially i created corners and creases at various area around my groins and hips. easy to create braces. so i was told to keep things round and soft and no creases, sort of like arc/bow shape.