What would cause a shudder under load

My guess is the driveshaft. It is the only non stock item and logically the first place to look. Having sold and installed a lot of rebuilt driveshafts over the years I have had shakes from initial installation to various times and mileages after installation. Issues have ranged from the yokes being slightly out of phase to my supplier trying a slightly cheaper u-joint in an attempt to improve his bottom line. It could also being something new like the rear main loosening up and the crank is flexing. There are lots of possibilities. At the mileage you have mentioned I would buy a new vrhicle instead of throwing money at a possibly hard to locate shake.
Randy