I'm looking for advise on Bilstein shocks. I still have the original shocks on my truck, which now have around 140,000 miles on them. It seems that most here gice a thumbs up for the Bilstein's. Which ones should I go for- the 4600's or the 5100's? My truck is not lifted or leveled and don't intend to in the future. I occasionally haul a 2 horse slant bumper trailer which I've had no problems hauling- it's a pretty balanced load. And most of my mileage is pavement. Are the 5100's a better shock over the 4600's due to the weight of these AV's? Are the 5100's geared for the lifted/leveled and the 4600's for non-lifted? I see most have the 5100's- why?