2003 2Dr Blazer ZR2 Auto 4WD

Rhythmic vibration between around 30-50mph. Increases and decreases with speed. Thinking it's probably bad u-joints and/or driveshaft. Shaft is badly scaled and probably out of balance anyway, so looking to just replace the whole thing, but I can't find the specs on it to know which part #/propshaft code to replace it with. GM Parts had 2 part numbers, both named "Front and Back" with no indication which was which. Can anyone point me in the right direction?

 

Rock Auto has a bunch of them listed by length. If you can measure yours, it shouldn't be too hard to get the right replacement.