[oil pan 4]

Originally Posted by aardvarcus

I also have a similar GM truck with 4:10 gears.
Is your tow/haul button pushed in? I don't think your transmission is shifting into overdrive, or the gearing is a lot lower than 4.10. Start from a dead stop and count the number of shifts it makes.
You stated 2200 - 2400 at highway speed, I am assuming that is your mentioned 90kph, or about 56 mph. If the speedometer in a GM truck with factory diameter tires reads 56, your are probably actually doing about 51 mph. (Your 265/70R16 is roughly the same diameter as factory 245/75R16.)
51 mph is 4488 feet/minute, and with 31” tires you will go about 8.111 feet per revolution, thus your tires spin 553 times in a minute. With 4:10s in the back, that means your driveshaft is spinning 2268 times a minute. With your transmission in direct drive (3rd), that would mean your engine is also spinning about 2268, so right in the middle of your 2200-2400 range. Your truck should not be in direct drive, it should shift into overdrive (4th) and have a 0.70:1 ratio, dripping your rpms to 1588, which would be about right.
Either that, or someone put rock crawler type gears in it, around a 5:86 ratio to get those rpms at that speed.

Sounds a lot like you have one of 2 problems.
Some one swaped out the gears or the transmission isn't going into over drive for some reason.
I would supect that the person who owned the truck before you realized the truck was having some kind of transmission issue and off loaded their problem onto you.

Another possibility is the torque converter lock up isn't working. But I think your RPMs would be closer to around 2000 or less.