A bad shudder

I’m having a nagging problem with my 2001 Astro 4.3. When the vehicle is under a load the engine shudders all the way back thru the drivetrain and feels down on power. At times I also get a rough idle that causes a minor shake. This doesn’t seem to change whether I back off the throttle or apply more under load. If I brake load the van in gear in the driveway I can’t seem to get the problem to duplicate, it only happens while moving. Van seems to run fine under cruising speeds once it gets going unless I have to press down the gas to pass someone. No DTC’s have been set or are pending. I was ready for a tune-up anyhow, so I changed out the plugs, wires, cap, rotor, fuel filter and coil (because I saw the side glowing blue). The parts had no effect on this problem whatsoever. I put a gauge on the fuel rail and I get about 58 psi on key turn, and around 60 psi running – even when the van is bucking under load. I also hooked up a vacuum gauge to see if I had any issues there or a possible exhaust blockage, but nothing seemed out of the ordinary on my readings. I was also able to temporarily swap out the cam sensor, crank sensor, throttle position sensor and ignition module from another running 4.3 to try, but none of those made any changes. I have noticed though that when the engine is cold the problem is almost non-existent, but as soon as I reach operating temp it starts happening.

Can anyone add some insights? Thanks in advance!

Is this an AWD model? If so, has the transfer case been serviced recently?

Well, I found and fixed the problem. Although this felt like an engine miss/stumble/buck because it only did it under load and low rpm, the problem in fact was a bad "u" joint going into the differential. Crawled under and shook the driveshaft and felt play, so I changed the "u" joint and the problem went away.

That certainly beats losing the drive shaft while cruising down the road.