Checking "Crankshaft end play" on engine while in vehicle ????

BACKGROUND INFO
- 1980 International Harvester Scout II which has a V8 engine (not sure which V8 version) and 3 speed manual transmission with 4WD transfer case
- fluid(s) dripping down from bottom of bell housing
- remove transfer case, transmission, clutch and flywheel
- replaced rear main crankshaft seal as well as input shaft seal on transmission
- re-assemble only to a point where the flywheel was mounted back onto the crankshaft
- run engine (with transmission still out of vehicle) while vehicle was still on hoist - no leaking from rear crankshaft seal

PROBLEM / CUSTOMER COMPLAINT
- customer picked up his vehicle and drove it home
- oil still dripping from bell housing

LIKELY ISSUE
- care was taken to install the rear crankshaft seal properly and the crankshaft seal did not leak when running only the engine while the vehicle was still on the hoist
- likely, I am dealing with "end play" in the crankshaft

IS THIS EVEN POSSIBLE?
- is there some way to determine "crankshaft end play" while the engine is in the vehicle?

As always - thoughts appreciated and thanks in advance.

Manual gearbox or auto trans ? on an manual transmission crank shaft thrust washer is quite common at high mileage caused by the end thrust of the clutch mechanism when the pedal is depresed.
Crank thrust washer wear is usually confirmed by levering the crankshaft pulley forwards and backwards with a large tyre iron or tanker bar.
Usual symptom is not oil leakage but very slow cranking when the clutch is depressed.

On high mileage vehicles excess crankcase pressure is the first suspect for oil leaks,