There is no need to pull the bell housing to replace the rear main seal. I just did a clutch job on my 87 FJ 60 and could not convince myself to change the rear main. The OEM seal looked immacualte and the replacement I had was of far lesser quality. It was a tuff choice to make, but so far so good. I may be kicking myself in the arse when it begins to leak one day, but until then.... Not a hard job, just go with an OEM seal and change it out.
Oil pan gasket leaks in the rear arch are also possible, as well as transmission input shaft leaks.. When diagnosing a leak in that area remember there are 3 possible sources. I suggest replacement of the transmisson input seal as well as the rear main.