Not quite sure your question...
Any suspension whether TTB or solid axle can be maxed out if you have a tall enough ramp, after which point a tire will always lift up off the ground (vehicle weight balance will determine if it's rear or a front tire that lifts).
If you're asking about something else, maybe you can rephrase?
Edit, rereading your post, I think you're asking about while on a trail rather than a ramp... If so, then that is a dynamic situation that makes it harder to say exactly why a tire may have lifted, however if the suspension (any suspension) has stiff springs on it (as would be the case with run-of-the-mill TTB lift springs), then the odds of lifting a tire while driving over a trail undulation increase proportionally. Weight transfer off the front axle while climbing a steep hill increases the odds of lifting a tire too.
In this picture the driver-front tire was just barely touching the ground (I could lift up on the bumper and raise it off the ground a few inches). Another inch taller rock and I probably would've carried that tire.
The guy behind me in the Jeep however carried his tire over a foot off the ground over that spot, entirely because his springs are stiffer, not because he has a solid axle.
(hopefully the pic loads, Supermotors has been flaky for over a month now

)
Do you happen to have a link to the thread ?
Unless there's another thread, it looks like he welded his TTB into a solid axle.
http://www.therangerstation.com/forums/showthread.php?t=91231