A lot of people identify the factory transfer case linkage as a problem maker. It is a ridiculous array of rods and levers, held together by bushings that inevitably disappear over the 14-23 year life of these vehicles. You may be able to test whether this linkage is the issue by holding the transfer case handle down during slower speeds. You may even feel the knock at your feet. If you do find this as the (or one of many) culprit, the most common fix is to replace the whole linkage assembly with a shifter cable.