I think you can replace the trans back seal without removing the trans. You will have to disconnect the shaft and take the trans output flange out. Do not forget to put sealant on the shaft splines when you put the flange back in place as otherwise it may leak through there. Leaks in the back can also come out of the speedo cable seal. If I were you, I would put the car on a lift and look precisely where the leak(s) come from. This may save you some time, effort and money.