Silent Runner gave some good suggestions.
I'd also add - make sure your driveshafts are inserted fully. Try to pull them out - you should not be able to pull it out by hand. The passenger side (right) shaft should be locked with the c-clip in the bearing retainer to make sure it's fully inserted, as it can be deceiving. I'm assuming the tranny is fully seated and bolted to the engine.
The spin one wheel and the other should turn in the opposite direction is a good test - give it a try.
Turn the non-rotating wheel and make sure that it spins the axle just fine all the way into transmission.
I would advise against jamming anything - the engine has a lot of power, and things could go flying.
To answer your question - no, it does not take all that much effort to stop a wheel from spinning. I was sanding a brake rotor once using the engine to spin it, and sand paper against rotor was enough to stop it.
With both wheels off the ground and nothing blocking either, both wheels should spin as long as the care is reasonably leveled. If it's tilted to one side, that is likely to prevent the other wheel from spinning.
Finally, it is possible that the diff itself is a toast, esp if you got the transmission of unknown origin, but that it unlikely. They rarely go bad, and when they do, you usually lose all power as opposed to just one wheel.
