I would not tow in OD (4th) the ratio is 0.694:1 and the engine really has to work hard. With converter locked you also loose the stall torque ratio which means no torque multiplier in theory but with extra slip adde I'm not sure I wanna cook the tranny either...I'm not sure what diff ratio is in yours perhaps 4.3 and given 17" wheels (275/65)
How heavy is that trailer?
IMHO, towing in 3rd or direct with lockup and scaling speed a tad down should bring EGTs down too. That would be approx 2700 RPM@90km/h (for 17" wheels/tyres combo) instead of sitting bellow 2,000 RPM in 4th(OD) which where it will labor hard hence high EGTs.
For 16" stock wheels/tyres (265/70) that would be 2,700 RPM @ 90km/h in 3rd(direct) or 1,925 RPM in 4th(OD) which is again too low and it will bring high EGTs.
Regards
Note: for anyone wanting to play with gear ratios etc ==> see this site, it has Nissan Patrols transmissions listed too....
http://www.grimmjeeper.com/gears.html