No you do not have to remove the engine to replace the rear main seal. It can be replaced in the car, although it is more difficult to do. When it comes to sealing the rear of the oil pan, I have to remove the engine to get it sealed properly (it's a rear sump pan that leaves no room between the pan and bellhousing). I recently had an issue with the standard Fel-Pro single lip seal. 3 seals later I used the Mancini double lip seal and this cured the seeping rear main. I will never use the Fel-Pro again. But you definitely need to find the source first. Use an oil dye and blacklight to find them easily. If it's leaking from a cam plug or something like that, then you'll have to remove the trans or remove the engine to even see it. But the dye and blacklight are very useful tools to find them pesky leaks.
