you may have one or two problems,, 9 out of 10 says your flex-joints are not the problem
1 the center bearing support is bad,, easy to fix Needswings has them just remember when you put the driveshaft two halves back together make sure the three notches line up there are two notches on 1 half and 1 notch on the other half of the drive shaft they are for balance .they should be
1 1 this is raised on one half of the driveshaft
-1 this is raised on the other.half of the driveshaft
your u joint is bad in the center of the drive shaft, hard to find a shop to repair(replace the u joint
here is the way i will be trying on my next drive shaft that goes bad.

replacing the u-joint in the middle of our driveshaft.
I have been working on this on and off for the last year or so.,,i think i found an answer.
1 measure all 4 sides of the u jiont death before you mess with the drive shaft
let's say you get 45 thousands from drive shaft housing down to the u-joint on all 4 sides no make 4 slugs .45 thick and about .5 thousands less on the outside so the slug will be pressed into each hole of the u joint (after the new u-joint is installed ,,,now with a press,,,press each slug into the u-joint hole tight and even (which will center the u joint ,,,,no do the other half of the u joint,,, once all 4 slugs are even tack weld each side,, now i think the u joint will be centered. jim