Look for the TC drop, I would be surprised if you had 4” lift with no TC drop and not felt some significant vibes along the driveline. The TC drop is simply spacers between the skid plate on the bottom and the frame. They just look like big black pucks. It is a cheap fix to driveline vibrations to fix pinion angles. Not sure an SYE will fix your shifting problems. Someone else may know on this thread (one of the best I have seen by the way) Is it an automatic? If so, you may want to look at your shift linkage for your transmission AND your transfer case. Both run down the driver side of the transmission and after lifts, they may be in slightly different positions. In an automatic, some time the shifter for the auto transmission will run into the linkage for the transfer case (4WD). This could explain your problem. To check, get a friend, chalk the wheels, set the brake, turn on your vehicle, and have your friend cycle through the transmission gears or try to put it in 2nd. If you lay next the vehicle (not all the way under it, you will see the linkage moving and might be able to see where it is binding. If it is an auto, this could be the problem. I ran into a lot of forums that discussed this problem early on in my exploration. If it is a manual, someone else will have to chime in.