[Sinclue]

All,
I wouldn't think that backing up onto a ramp or staggered blocks (like the lego type) would put that much strain on your transmission.  Now if you are trying to push it up over 2x4s that wouldn't be good.  But I've been in sites where there wasn't room to back in far enough and then pull forward to level.  And sometimes you have to back up an incline into a level-able (new word) spot. We use the lego type blocks.  Lightweight, easy to pack, and they dry quickly (unlike wood blocks). Boondocking can put you into some interesting and challenging situations.  
I certainly wouldn't recommend any situation where either of your wheels wound up hanging in the air.  Much discussion has taken place elsewhere about using the jacks as stabilizers vs support.  But I wouldn't want to depend on just those two bolted on spindly things to support my investment...no matter what their rating.  Its designed and built to sit on those wheels and I'm a firm believer in letting them do their job supporting the trailer.
Jim