I wanted to update this thread in case anyone else, especially newbies like I was, come across it.
The following year I discovered that I had a cracked head. It was likely cracked before the Kamloops trip, and the leak was very slow. I have since redone the heads and the machine shop that rebuilt them for me actually couldn't find the crack with pressure testing. They knew it was cracked because I showed them the evidence, but they wanted to make sure they were rebuilding the correct heads, and not the one that was cracked!
Anyway, the coolant slowly seeping into the engine oil was causing the sludge I mentioned. I'm not sure if such a slow leak would have caused the engine temperatures to rise, but I have learned since that it is very common for engine temperatures to rise on the Coquihalla as Kwantau noted. At the time I only had the analogue dash gauge for temperature. I knew it was rising, but I didn't know the exact values as those gauges can be rather inaccurate. I was probably still in "safe" ranges, but if you've ever driven the Coquihalla, there is not much room to pull over if you overheat. Plus, people are doing 120-130 km/h (75 - 80 mph) on that highway. In comparison, the Grapevine which we did a couple of years later, has wider shoulders and trucks/trailers are required by law to stay below 60 mph. I felt much more comfortable on the Grapevine than the Coquihalla. Next time we travel to the BC interior I'll be taking the Trans Canada or another route.
I now also realize just how much transmissions play into all this. Our Tahoe has a 4 speed transmission. Compared to our Explorer, with its smaller engine but 6 speed transmission, it feels much more capable on hills. The 6, 8, and now 10 speed automatics are making a big difference in towing.
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