That lip around the bottom of the block/oil pan will collect oil from a valve cover leak, could also be the dip stick tube oring.
I wouldn't bother with an aftermarket rear shaft except maybe Tom woods, I tried an aftermarket shaft with spicer joints and haven't been impressed, after 2000km I have play in the centre ujoint, seems out of balance out of the box and had no grease in the centering ball upon receipt, the centre ball requires a needle greaser and is not in a great position to be greased, it has cost me thousands in parts and days/weeks of my time trying to narrow down a bad new aftermarket driveshaft and seller offers no support or warranty as it would have to shipped back to the states and I've exceeded my 90 day warranty replacing other parts narrowing it down to the aftermarket driveshaft even though I requested a replacement the same day I got and returned it and gotten same shaft back greased... I Just ordered a OEM shaft at $1100 CAD and the aftermarket was $700 so that alone has cost me $2000 after taxes no labour luckily, but lots of beer and cursing. Use your VIN to get the right length shaft, there are 3 different propeller shafts for different tcase and trans applications.