Wheel slip is something that I knew would happen in various scenarios, and for dead-reckoning it becomes important to at least apply a guess factor in known situations.
As it happens I was surfing youtube and discovered a video from the JPL Mars Yard that I hadn't seen yet (!) and answered a question on wheel slip when obstacles are involved. Here it is:
The important bit happens after the front wheel comes down off the rock, starting around 0:22 seconds. If you watch carefully the wheels are apparently driven at the same speed, I think. The middle wheel mounts the rock, but the front and rear wheel are held back and slip quite a bit.
I was thinking the correct thing to do would be to drive the middle wheel faster, since it has a larger distance (over the rock) to travel, but it looks like given the slip involved to drag the middle wheel 'up and over' the rock, it's simpler to drive them all the same.
At least I know when to expect another wheel slip situation, and for dead-reckon driving that it might happen more often, i.e. when the suspension feedback is indicating that a wheel is mounting an obstacle.