1stGen SR5 Disc's squeal only while trailering

Thx for stopping to read. 1stGen SR5 Disc squeal only while trailering [squeal is at about last 5'-10' of full stop], otherwise quiet braking w/out trailer load.
Just acquired this '06 Tundra 8mo's ago and this is the 1st tow with it. Boat and trailer weigh about 4800#; front trailer axle has hydraulic surge discs.
Squeal is noticeable b/c truck stops silently otherwise when not towing [squeal only occurs at about last 5'-10' of stop].
Since day of purchase, brakes seemed sensitive compared w/my 1stGen Tacoma SR5, 2WD, but Ive gotten used to them and have stood on them a few times as a stress test and they been good. Have no idea on age/disc-pad measurements. I might get a caliper on the disc w/out pulling tire, but I'll wait til next tire rotation to get good measurements on both.
My guess is it may be due for at least pads?
Anyway, no problem stopping trailer-boat load otherwise.
Bkground:
Since I purchased Ive replaced the LBJs, OuterTieRods, EndLinks, Sway bushings, Rack bushings and boots, Bilsteins x 4, brake flush, tires [265-65/17 HwyTerrain] and truck drives great street to highway.
Overall no complaints for a 19yr old; runs, stops and handles like a 2 yr old.