Occasionally my 98 will do the shutter thing. Usually when going up a steeper than normal grade and I'm hitting the hill slow such as in town driving. It almost feels like the front tires are turning a tad faster than the rear. I have the limited slip rear but this feels more like it's coming from the front. Since it has the auto 4x4 system and it only occurs at slower speeds (under 35mph) I'm almost certain it's a side effect of the "traction control" system being implemented under 35mph. Once 35mph is reached, the vehicle is basically in 2wd and the shutter is not there anymore. I don't see any abnormal wear on the front tires, so other than maybe wear on the transfer case clutch to the front dif I believe this to be normal.
The way I understand the control-trac system is from 0 to 35 mph there is 5% of the driveline power going to the front tires. This changes with the position of the steering wheel to prevent front wheel hop in corners. If rear wheel spin is detected, more power is transfered to the front in step until the rear wheel spin has been eliminated. Under heavy acceleration, more power is transfered to the front regardless of rear wheel spin. This could be why we feel a shutter because there is more power to the front end and in fact the normal front wheel slippage is causing the front wheels to turn slightly faster than the rear causing the shutter we feel.
Now after saying all that (ok I typed it), awhile back on this forum (and can't seem to fine) someone was told by Ford that the transfer case clutch needed to be "zeroed" to eliminate this shutter. Unknown if this is the case, I have never asked Ford to check it. My shutter did seem to me minimized after getting the latest Powertrain Control Module reprogramming recall performed. I don't know if this zeroing is done during the reprogramming but the shutter is there but not as noticable. Basically it's there only when going up hill and heavy acceleration is required.
Hope this helps.
