It's great that you are taking the time to review your TV weight limits in conjunction with your selection of a trailer......., it will save you a whole lot of towing disappointment later on.
If all you can find is reference to "weight carrying" mode, this implies that either your TV or TV receiver is not designed for a WDH as you referenced in your first post (doesn't mean it can't be upgraded). Normally the TV's receiver will have one, or both weight modes identified.
I did find that a 2010 Highlander with a 4-cylinder has a 3,500lb towing capacity, and the 6-cylinder model a 5,000lb tow capacity. I can't find any reference to any "weight distribution" mode capacities (doesn't mean there isn't any). At the moment it appears that even in "weight carrying" mode your TV can tow the X17Z, but you would have to watch your actual loaded tongue weight. But the obvious advantages of using a WDH is worth looking into.
If you have the 4-cylinder model, the X17Z is definitely not a good candidate.
If you have the 6-cylinder model, we need a little more information about the TV's ability to use a WDH, and this will aid in confirming what potential weight issues you may or may not have. Don't forget, depending on how one loads the X17Z, the loaded tongue weight might be manageable.
Keep in mind the 5,000lb tow rating is based on the TV's "curb weight", so the weight of the TV's options, passengers, and cargo must be subtracted from the 5,000lbs, this will give you the "actual" (real) tow rating. Even so, you still may be ok with the X17Z, just need a little more info.
If you have a local dealer with a Jayco X17Z on the lot, find out what the yellow shipping sticker states for the "actual" ship weight.
Here is a reference thread on a situation similar to yours but with different weight issues, but the weight review "do-diligence" is the same. http://www.jaycoowners.com/showthread.php?t=3800&page=3