Intermittent Shudder

2002 GMC Envoy SLT
4.2L I6 AWD
210,000 miles

When driving at speeds between 50 and 70 mph at a constant rate the vehicle shakes. If I immediately let off the gas or shift into neutral it goes away (but will happen again periodically if I let it get back to a constant RPM). It happens more often when it is in the 2 wheel high setting. If I change it to A4WD it still happens but much less often and the shaking isn't as hard.


The problem seems to occur when the road changes from downhill to uphill. I think it could be having trouble shifting from 4th back into 3rd order to maintain its speed when going up the hill.
If I accelerate and then let off the gas and then accelerate again (never being at a constant speed) it doesn't happen.

Check engine light is not on.
Took it to autozone to have them hook up the scan tool and it came back with a "C0237" code and that's the only one.
I have inspected the u-joints and they do not have any play or signs of damage.
The transmission fluid has been changed regularly along with the oil and filters.
My thought is that the problem is either the transmission itself slipping or the torque converter clutch slipping.
Does anyone have any insight?
I am by no means an expert and would appreciate any comments or suggestions.:1zhelp:
Thanks in advance.

T diagnose this from here is almost impossible, I have the same truck, so am familiar with it's operation. Many things can cause this, if the entire truck shudders this could be drivetrain related, even a misfire under load can cause this. Bottom line someone who is good at road test diagnosis should drive it for a better feel of what is happening. If it only does it in overdrive under load, this is usually an issue in the ignition system, or the engine is too lean, low fuel pressure, dirty injectors. That said it could be the torque convertor, like I said a thorough road test and live scan should be done.