Correct me if I'm wrong, but I believe GVW ratings are relative to a load being placed over or in front of the rear axle. When you extend your underlift, you are transferring weight OFF your front axle depending on how much extension you have. With that being said, I have owned quite a few WL trucks over the years and I prefer 161" WB on LD and ALWAYS beef up the rear springs. On one 1-ton I built, I added 3 leaves to the main and 3 leaves to the overload - I think I had a total of 14 leaves on each side. I could tow box trucks from the front with no problem with plenty of steer axle weight - the W/L of course was mostly sucked in. I now have an 02 F550 12 ton 161" WB and I also have no issue with it as far as steer axle weight goes. All the responses to your issue have indicated that it is a given to beef the rear springs on a wrecker. As previously said, this will due little to offset front axle weight relative to W/L extension. Hope this helps - thanks, Steve.