This week I replaced the lower gasket on the rear lift door....it is part 68281B and includes the body clips that hold it in place in purchase price. Toyota is VERY PROUD of this small gasket....it is $146.
Reason for change.....I found some rust about 2 inches to left of the rear lock on bottom of upper lift gate door. Search of mud showd rust around the handle that is used to open liftgate in rear but mine is perfect there.
At first I thought it was from the rear window washer tube leaking down as it does not work (only thing on truck I can find that does not work) and rusting from inside.
Removal of rear panel to upper door and an inspection inside revealed it was bone dry. The rust was from outside...and there was one pin hole of where it made it all the way through. Crap!!!
So I started thinking and wondered why SO MUCH WATER was between gasket and door itself when it had not rained in couple days. Made sure could see nothing else but with it coming from outside I was able to rule out window glass and such leaking into body panel.
My only guess was the gasket was old and no longer fitting tight as it should against tail gate and I was going to order a new one...clean area really good...wire brush rust area...treat with rust conversion...and cover with Rustoleum white for now to seal back up and see how it goes.
So I ordered the gasket and after I picked myself up off floor from price I also ordered new rear tail gate support cables. One of mine broke when I was lifting a deer I shot into back of LX this year...so replaced both before it broke two and slammed tailgate onto bumper.
Anyhow new cables are easy to install... No pics taken...just have proper Torx head socket....remove old set...install new set....DONE!
Gasket removal was easy...used flat door panel tool in case new fasteners were not included as they are like $2 each from Toyota. They were included and may be one reason the gasket is so much. Once I removed it had about a teaspoon of water come from gasket. Thought it might be inside gasket where air holes were clogged but all were open and water was actually forming or setting on gasket itself between body and gasket.....as if it was running down and along gasket and pooling.
Wire brushed rust.....treated with rust conversion and also poured some from inside so it would go through the one small hole smaller than poppy seed. Did this three times to ensure complete coverage and let dry....when turned black. Covered with some Rustoleum gloss white....little over spray on lock bolt but compared to rust that is not big deal to me. Got it looking good....or better.
Then I wiped down door completely once dry....cleaning it and ensuring no dirt, gunk or anything. I also saw that the ends of the gasket had left a residue on door...and surmised they were meant to STICK to the corners....but no longer did. I wondered if they were catching water and allowing it to run down length and started the problem.
New gasket I started at each end...pulling sticky cover off the corners and then putting into place...working around area pushing in the included and mounted attachment clips and gasket. Went around and pushed up tight.....and shut tailgate. It did not shut good first time and a week later I still have to make sure I shut it hard enough or want shut all the way. So I know the gasket is resisting and forming to tail and lift gates to seal.
It rained good the other night and next morning I checked...and bone dry....no water between gasket and lift gate anywhere....hoping this was my problem.
Pictures show gasket itself.....it goes on bottom of upper lift gate..and finger points to area of rust...it wraps around under the gasket and would have been hidden if it did not spread enough to see. Wish I would have taken pics before and during wire brush but that is the breaks.
Check yours for moisture settling and remaining there...and for loose end corners. Unless I had a scratch or something to compromise the paint it had to be doing this for a long time to work through wax, clearcoat, paint and start to rust....worth checking.
Just incase you decided you wanted to post some updated pics of course. 
