After wading a few drive shafts up i need to know how to figure my critical speed or some thing of that sort so I can have my carbon fiber one made. (4L80E), 4.10's, 29.67" tire, my flat to flat measurement is 56.125". Ok, now on to what i have thrown out 3.5" .090 wall DOM steel shaft, 3" chromolly .065 wall, 4" .120 wall aluminum one so i guess what im asking does any body know what my critical speed is or what else is needed to help me fix this isshue? Dont really want to spend $850 on a carbon one if i can make do with some thing else.