the factory radiator has ports for oil/transmission fluid cooling purposes, so that's why you see the lines going from just before the filter to the radiator. the auxiliary cooler is probably not in the best place it could be, I used to have mine right under neath the license plate laying close to horizontal to the ground in between the front air dams. just something to think about if you ever wanted to move it. when I got a bigger front mount intercooler I upgraded my trans cooler and moved it to the rear of the car.
as far as the air bags in the rear springs, you are able to replace just the lines. if you find its the bags and wanted to replace them, I've got a couple new ones in my garage that I could sell you for less than they cost me. I mounted them but changed my rear spring set up completely and couldn't use them any more.
for stability at high speeds, the shocks and springs would be my first guess. or control arm bushings, as mentioned. after that would be control arms, for the rears I would only go with one company and that's baseline suspensions. their set up changes the angles of the control arms in a more favorable manner that aids in getting traction off the line. do a search here and read up on their site as well, lots of good info to read up on so you can truly understand how it all works together. for the front control arms, lots of people have opinions about one company's product versus another but at the end of the day just make sure do your research here.
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