Updated info. on shutter/vibration

Ok, I posted a while back about a shutter/vibration coming from my truck at very low speeds (at just above an idle) while turning. Several people suggested that it may be the limited slip rear end chattering. Well, I think I have isolated the symptoms today while driving through a parking lot.

I'm pretty sure its either coming from my power steering pump or brakes (don't really think brakes though since it won't do it while moving in a straight line). Either that or turning the wheel is causing a shutter in my drivetrain for some other reason (pressure put on a u joint, etc.). Anyway, here's what happened today...

Pulling forward out of my parking space (which I am backed into)- start truck, put into gear (auto), idle off just a bit while still holding the truck back with the brake, turn steering wheel a good bit but not maxed out, and the shutter/vibration starts. I can feel the shutter as if its in the power steering pump/steering wheel and also through my seat. I let the wheel straighten up and the shutter stops. If you apply the break the rest of the way and come to a complete stop, even with the wheel still turned, the shutter stops. Its like it shutters when the wheel is turned and your holding the truck back with the brake to prevent it from moving at full idle speed.

I then tried to make it shutter with the wheel straight simply by applying the breaks at various low speeds until eventually stopped and it won't shutter. I then went to another store and it shuttered while I was backing out of a parking space since I was on the brake and turning the wheel at the same time. I also noticed that once you give it some throttle and you get just above idle speed, it will stop shuttering.

My powersteering seems to work fine and it is not hard to turn at all. No squeals or audible whines. Sometimes at lower speeds it does feel as though the steering wheel is rubbing something while turning, but its very slight and not a problem.

Any idea what this is? I do have a lite oil leak from the vaccum pump but that the only issue that I know of. Could this be a result of the vaccum pump not doing its job properly due to the leak? If this is being caused by the power steering pump, does this mean that its likely going out? Any advice is greatly appreciated.