With the transfer case apart, we could see how the internal components were wearing. Inside the transfer case are two shift forks. Both have wear pads, and these pads need to be inspected closely. If they're worn, the transfer case can be hard to engage in four-wheel drive and/or 4-low, and/or they can cause the unit to pop out of gear during use. If they're worn, the shift forks are replaced with new units. Ours were in good shape, but we replaced the four-wheel-drive fork with the new unit anyway.