Bumping this old thread since I just had this same experience with my 2006 Sonata. Installed a new Trakmotive aftermarket axle (LH) and it shuddered terribly when accelerating between 25 and 40mph. Also had a pronounced vibration at certain highway speeds. Took the car to a shop, and they said it was probably a defective new axle. I had them remove the axle and try another new aftermarket axle. Same exact problem! So they took it off, and tried a THIRD new aftermarket axle and guess what? Yep, same issue!
So here's what solved the problem...
Gave up on the garbage aftermarket axles, and tried a Cardone reman. And wouldn't ya know it ..... problem solved! Apparently, the good reman companies like Cardone use OEM cores only. I don't know if it's mostly Hyundais and Kias that are having this issue with aftermarket axles, but I have never seen this with any other make.
So, either 1) the aftermarket companies came to an idiotic consensus that Hyundai/Kia axles are the same as some other OEM's, or 2) some douchey company like Toyota gained a controlling interest in popular aftermarket parts manufacturers, so they could basically try and screw every Korean vehicle owner. Yeah it's most likely #1, since #2 is quite a stretch and strongly appeals to conspiracy theory (although nothing seems to surprise me these days).