I had MorRyde SRE4000 installed a year ago on a 9400 lb. fifth wheel trailer. Night and day difference--eliminated 98% of the trailer chucking. I believe that MorRyde recommends IS for 7.000 lb. axles and higher. They recommend SRE4000 at the 6,000 lb. axle level. I just had SRE4000 installed on 6,000 lb.axles by ordering on line and installing through a less expensive axle shop. Feel free to PM me for the name of the shop. MorRyde is very good. Only two problems: 1) they schedule very precisely and are currently booked out 3 months in advance and 2) their shop rate per hour is pretty high. However, no one will be disappointed with the service work.