I have a stock '78 FJ40. I'd like to install the 4" Skyjacker lift kit, retaining the stock length shackles. Is a 4" lift enough to cause drive shaft issues (binding, too short, etc.)? How high of a spring lift is possible before you run into issues with the driveshafts? Is the 2 1/2" lift a better choice? I just don't want to spend the money to install the kit only to have to spend a bunch more money to correct any problems it causes...