You don't need to re-torque the pinion nut as long as you mark it berfore you take it off. Use a center punch and mark the nut and stud. When you put it back together, re-align the marks.
Follow Dusty's method to loosen. I would use a big pipe wrench and cheater long enough to brace against the frame. And it won't hurt to heat the nut with a propane torch.
