Well, here's an explanation I got off the news groups. It makes perfect sense to me since I don't have lockers and have fairly limited flex. I bet that when I have somebody watch me during this "lurch" they'll see the in-air tire spin backwards for a sec until it's on solid ground again.
Gotta work on that flex and get me some lockers. What locker goes into a stock D35c without backlash adjustment and special tools?
This sounds like the Lurch. Here is a good quote from the following article
that describes what is happening.
"The lurch happens when the two tires that are giving resistance on the
descent slide or catch air. By now we have the understanding that 4WD is not
4WD unless you have lockers. So, there are usually only two tires holding
the rig back. When they hit the loose stuff or find air, the other two tires
which are static and just rolling along will not help to keep the rig from
moving forward very quickly. This "lurch" forward a few feet is a bit
unnerving unless it is anticipated.
On loose trails, wet, rocky, sandy, soft slick rock, etc, i.e., almost any
trail, the lurch is a common thing. The thing to do is not panic and jam on
the clutch or brakes like at a stop light. When going through such sections
expect it and drag the brakes lightly or engage the lockers."
'91 YJ, I-6, 3" lift, 31s