That shudder you have, I have it to at the same speed. Its can be a few things, but i dont think shocks is one of them. I replaced mine in the last thirty days and I still have that shudder, and I'm using Gabriel ProGuards up front and Gabriel Load Adjusting Shocks in the rear. So I have a really firm ride. I checked my ball joints and control arm bushings and found out it all needs to be replaced. You might have the same issue. And if you replace all those parts and still have a shudder, it may be one of the axle/hub nut has backed off a little.
As far as having a Lincoln like ride, OEM style shocks may be the way to go. Rockauto had Motorcraft shocks for about 30 bucks a piece. for both front and rear.