Need help, before I break somethin'!!

I take it you you can shift smoothly from low to high range and visa-versa....

The key is to preselect the next gear or split before shifting or splitting....

So....
6th Drive.....Preselect to OD then let off the throttle.....you now should be in 6th OD...
Then preselect drive....throttle off.....move the stick into neutral......then ease back to 7th drive and ease in the throttle at the same time...Rinse repeat.....

Do you know how to "Progressive shift" btw?

And why do you shift at 2100?....There's no need to bump the governor....even on a kitty....

 

Unless you are driving a Cat engine that is older than 15 years or so, you can lug them down to around 1200 before you have to shift, and I would upshift way sooner than 2100!
The over drive is like a half gear shift 300 or so rpm instead of around 600. So when downshifting from 12th which would be direct, to 11th which would be an overdrive or the high side of the splitter, you would have to rev the engine 300 rpm to downshift.

 

You are shifting way too high....That's unnecessary wear and you are wasting fuel....
What are you getting MPG wise...about 2 MPG?

 

It shouldn't bog down or lug as it were.....

The power-band on that engine should be about 1200-1500 IIRC.....

I suppose you don't know where "peak torque" is on that kitty right?

Look on the data plate on the valve cover......