Ok, this may not be the kind of advice you want but I hope it helps...Part of the problem is the range stormers excessive weight...Do you have the stage 3 weight reduction installed
Also this vehicle is AWD ( I think )...That could be part of the problem as well...
Anyway to fix understeer keep the ASM and TSC as low as possible because they induce understeer. You can also adjust the VCD to throw more of the cars power to the rear wheels...( however this may cause oversteer...but you're an experienced GT4 drifter so that shouldn't be a problem )
Also you can try adding negative toe to the front tires and positve toe to the rear tires.
Does this car have more than 450HP

If it does not than DO NOT use LSD because it too induces understeer!
A lower front ride height and higher rear helps also. Thats all that I can think of now...If you have any questions you can post back, or go to the tuning guide located in the navigaton section of every page.