[spdbrake]

Changing the diff side seal in the car can be done but mucho easier with the diff removed. I'd make real real sure its the flange seal and not a loose axle (six attaching bolts) allowing CV grease to sling out.
If it the axle they're pretty cheap new thru Rockauto @ $60 per.
Typically the rule of thumb is rag-wrench a seep and fix a leak. Both are subjective though.
Plenty of youtube videos on the subject. Search for E36 differential seals.
ECS tuning and Turnermotorsports sell complete diffs and seal kits.

Search for 1995 325i for parts.