screw coils, it's too much work and there's no reason to go that route. leafs are tried and true, and will flex fine if you get the right springs. like BiggieTruck says, the diff is moved way over to the tire on 85+ trucks because they had to clear the damned TTB crossmember, and there is no room to add a coil seat without doing some major fabrication that would involve welding to the cast steel center section. build your own shackles or get some non-OEM ones so you can ditch the tracbar, which I would definitely do if I was dropping a grand on an axle.