I went out for an off-road week end. (will post the pics asap

) and the KJ started over heating since the first uphill trail.
The temperature gauge went past the mid part (vertical), and a few moment later, when the gauge reached the 3 quarters, the temp.Trans. cell went on.
So i stopped for a while.
Its the first time the KJ heats up since I own it (it has now 106'000 miles), and i did exactely the same trails last year with no problems.
It heat-up every uphill path during the week end. I noticed also that the best engine speed was around 2'000 rpm in 4th: Best power and less heating. But still, i had some heating up and had to stop now and then.
My question is, what would cause the KJ to suddenly heating up like this, it never did before, and I really did not push it at all.
My buddies insist on checking-out and cleaning the dirt that could be between the coolers, as I did quite a bit of off-road lately.
I will also have the coolant flushed out.
But beside this ? what else ?