The fact that this is quiet now when damp is interesting and points to something that is lubricated just by the damp weather/rain even. The tailgate parts Jim mentioned AND all the things where the gate contacts the body of the bed should just plain be lubed. Much of this you could and should do yourself. There are lots of things throughout a vehicle that need to hinge, pivot or just move some with body to frame torque (bending/twisting) and parts that "hang" the exhaust etc., etc. The above pic is an assortment of items used.
You said you noticed the tail gate was getting more difficult.... great place to start!
I would get yourself some WD-40 (I like that by brand name) - some spray, white lithium grease, spray silicone, and a plain old oiler like in a can available at any decent auto parts or even general stores like Wally World.
I'd (and I do with all my own stuff) go around and spray the parts that move with the lighter sprays first. Heavier metal items that are strong hinging I finish with the spray grease. Do this to all the things you can find using care not to spray on hot parts, or parts that you don't want messy, painted parts that can be seen might react to some lubes so careful there, DON'T SPRAY ON BRAKE FRICTION SURFACES, but keep looking for all the locks, hinges (doors, hood, tailgate etc.,) all around the vehicle.
Just for laughs I'd do the tailgate first and see if that isn't the immediate culprit - dang well could be. Spray the WD-40 inside the latches in the cracks and the center handle on the gate - really get some up, over and around in there and the tabs that lock it shut. Those can be a problem and don't latch after a while and sometimes need to take things apart to lube things.
As for manuals: Last time ( ages now ) I checked them out they were mostly basic and lacked exacting details but all had some good universal reading. Perhaps a library or book store would have some copies to peruse to save on some waste for exactly what suits you. Lots can be found on the web and there are sites like this for specific questions and problems (of course this is the best!) for close to live help as in here which is a fading thing on the web it seems.
Side note: Mechanical aptitude is NOT gender biased by nature!
T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/