2000 blazer (almost 200K miles) loses power after 15-20 minutes at highway speeds but regains power after stopping for a few minutes. Also has low pitch humming sound coming from rear of vehicle? Sound persists while moving or stopped. Could this be a catalytic converter problem or maybe a fuel pump?

Radiator cooling fans not working? The ECM is protecting the engine by shutting it down.

its a pulley, not the belt. most likely the idler pulley. the bearings inside are going up not a hard piece to replace and the pulley is around twenty dollars at advanced auto parts

driving in 4wd on hard surfaces produces what is known as torque wind up as the wheels cannot slip so the variations of distance going around corners , dips in the road etc place considerable strain on the transmission , transfer case , drive lines and diffs
that is why it is difficult to get out of 4wd unless you drive with 2 wheels on a grass verge to relieve that torque wind up
as for the noise check wheel bearings and the cv ("U") joint at that wheel as there appears a proble where the dog teeth in the hub are not engaging properly or are jumping out of engagement

I would start with plug wires rubbing on something and created a short.and is not compleatly worn through. a cheep place to start. good luck with it.

ignition modules can cause problems like this also airflow meters and faulty fuel pumps

Are there any codes in the pcm?
There are quite a few things that could cause this;
Make sure the battery cables are tight and clean.
If you have a way to do it, need to find out what the fuel pressure is at when it is acting up.
Dose it miss at all or just loose rpm's and die, no matter what you do? (meaning can you keep it alive by pressing the gas peddle)

To me it sounds like your new fuel pump is loosing pressure while running for a while. But try and get a pressure reading and post back, that will give us something to rule out, or to address.
I think the light flicker you are talking about is just low engine rpm's?

Your ac pulley may be worn so the engine needs extra boost and power to run it thats why when it is in optimal power (highway-- high engine power)as opposed to idle(low engine speed) it runs. You might need it replaced.

injectors

At idle the oil pressure should be at least 20# at highway speed it should be at least 30# these pressures are on the low end higher is better.

hi yes if its registaring 10 mile out it will be sending wrong information to the gearbox replace it with one that works yates210456