Well picked up a clean 03 with the 4.7 and towed with that this past Sunday. No issues at all. Left plenty of space between my vehicle and the one in front of me. Went at or under the speed limit.

My 3rd gen is now up for sale

Sad to hear such but ultimately the wiser move if you will be trailering the boat frequently. Enjoy it.

A 99 4R 4x4 is only 3400 lbs? A GX is a lot more than +1000 lbs. My stock 05 GX with KDSS (no hitch) hit the scale at 4950 lbs. with near empty fuel tank and no passengers. I towed a big block 22' Sea Ray that scaled almost identically to your Chaparral with a 91 Pathfinder 4x4 manual trans. (3.0 V6 w/ 180 ft. lbs. of torgue, around 150 h.p.). I think that was around 3500 curb weight and it towed fine. Never felt like the tail was wagging the dog, as it did flat towing my FJ40. Did a number of mountain pass trips from NV up into western Canada. It was pedal to the metal a lot, but it worked. Actually got better mpg at higher altitudes even though powered output was clearly reduced. I think you'll be good if your 99 is as light as you say ... I don't know, but I'm skeptical. GX should do even better. Enjoy!

this is an old thread and the op has oved on....but the info could be helpful for others...so I will chime in too.

I pulled a 22 foot SJX boat with an inboard, and loaded for a week of camping,, and pulled it for about 800 miles, round trip , including hills, with my 2000 4runner. It had the TBU at the time. I dont think it was a very good idea, I did not like the way it drove. It was a long planned out trip and the rig that was supposed to pull it was in the shop. They are not as heavy of a boat...but I still did not like it. Ive pulled many other things with my 4runner....smaller/lighter things....with no worries SJX Jet Boat Home Page - SJX Jet Boats
However my sons 2016 v6 taco pulls that boat with no issues and feels fine driving. But side note , sold that boat yesterday and going smaller with an outboard.

I have an '04. I dragged a semi tractor out of a ditch with it, no problem. It's 2 wheel drive 4.7 and that 4:10 rear... not much it won't do But it WILL use some fuel...