Fluid in rear axle flange coupling?

The oil is probably from the gearbox. The bolt in the center locks the splined flange on the output shafts. Some oil can bleed/slosh out from there. Sometimes you will see those bolts with RTV on them and some boxes us O rings too. Do not put oil in there upon reassembly. You just want clean dry axles (rebuilt or regreased). This is a place where you want to use an accurate torque wrench to put the flange bolts back in. You want to snug them all by hand and criss cross tighten like a lugnut and increase torque until spec as you criss cross around. You can have a leaking bearing where the axle flange goes into the box. That is evident by oil there behind the flange on the gearbox case. No oil don't bother. There is more flange leaking like you described and triple seal leaking with over filled gearboxes. I never fill to max. I always fill just a bit over the min. In my hands this seems to reduce triple seal problems and the axle flange weep.

 

Well yes you have no clue what the torque is. There is a lot of energy going through there. Maybe I'm paranoid because I have seen the damage axles can cause if they rip loose. Once one bolt fails they rip out like a loose wheel. As a newbie, "god protects the unaware" so you got that going for you. But since you are diving deeper into your car also invest in the tools you need and it is almost always better to spend the time to remove more parts to gain better access. If you stay in the hobby tools are an investment in "good outcome" from your diy service.