328: running engine/trans while car on jack stands...

I am doing various work on my 328 while on jack stands (doing many things: shaft boots, suspension bushes. shift shaft seals and some body work); the wheels are off.
I ran the engine while on jacks to heat up the coolant system for checking a leak experienced earlier while on the road (likely a bad coolant cap). I also placed in reverse while running to inspect for noise, etc and to load the engine a little. When I stopped, I noticed some gear oil leaking from the rear of the engine, from the rear engine mount area. Are the axle shaft seals stressed such that they leak with the car unloaded and the axles rotating? BTW, the leak stopped after running.