All had some loose parts in the drive lines and many lost their carrier bearings. Mine has had loose splines sense new and have gotten a little worse over the years. Can always pull the plastics and check but a quick test to see if it's in the drive line is be moving in 4wd, preferably up hill so you can load it, monumentally take your foot off the throttle and reapply a couple of times. If the vibration changes..ie gets worse or less when the power is reapplied, its likely in the drive line or..in a CV joint. If no change, it's probably in the differential or..more commonly, the diff is coming loose or has broken a mount.