The key difference between the Bilstein Heavy Duty vs Bilstein 5100s is that the Heavy Duty or 4600 series is designed for stock height trucks and the 5100 Series is built for lifted trucks. The other major difference between the Heavy Duty and 5100 is that the Bilstein 5100 Series shocks have a zinc coating with a brushed metal shock body and a black boot while the Heavy Duty uses a painted yellow body with a blue dust boot.
The Similarities: The 5100 series and the Heavy Duty shocks use the same technology internally. Both use the same internal valving and at stock height will ride the same (shocks for lifted trucks may use different valving settings in order to accommodate the extra weight of larger tires). Both shocks are pressurized with 360 psi of nitrogen to prevent shock fade and foaming of the shock oil. The outer construction of both shocks is identical as well with an extruded metal body rather for maximum strength and top of the line seals. The 5100 series and Heavy Duty 4600 series are both covered under Bilstein’s limited lifetime warranty that protects against any manufacturer defects for the life of the shock.
Stock height trucks - Bilstein Heavy Duty yellow and blue shocks are the most popular choice, because they typically cost a few dollars less than the 5100 shocks and provide excellent performance for towing, light off-roading or normal driving. Many customers do prefer the 5100 Series even on stock trucks because the shortest 5100s are typically built for 0 to 2 inches of suspension lift, meaning they can run the 5100 on their stock height truck but leave the option open for a lift later on down the road.
Leveled trucks - Bilstein has shocks designed specifically for a 1-3 inch leveling kit. The 5100 rear shocks should be used along with your front 5100s on leveled trucks because the rear 5100 shocks are usually an inch longer than stock to accommodate a small lift. If you already have the 5100 shocks on the rear, you always have that option without needing to buy new shocks when you decide to add the lift.
Lifted trucks - Bilstein 5100 series is the only choice. Bilstein 5100 shocks are designed for every size of lift for popular trucks, so whether your truck is a modified daily driver or an off-road truck, there is a Bilstein 5100 shock to fit it.