Usually vibration is either something out of round, or something worn out.
To check the drive shaft, you'll have to crawl under it and physically grasp the joint to check it for movement.
There are Three places to check; the two ujoints, and the crimp between sections. If the crimp looks like it's moved, it's your problem.
I was sure the driveshaft on Lazarus was fine; I just was upgrading to Aluminum when I took it out during an (unneeded) transmission replacement.
It was in two pieces, and rattled like a maraca.
I thought the tranny was slipping in second, but noooo; it was the driveshaft turning inside itself, lol.
We had a group buy on aluminum driveshafts a ways back, from PST; call Arlene, she will hook you up.
I have three of them, one over 100k, with no probs at all.
Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
Black '96 Cougar XR-7 (Lazarus) 210k mi PI Intake, '02 4R70W, Jmod, PST DS, GrogTune, Konis, Mark LCA+Poly, racecougar Custom Engine Chain, and JL and racecougar Bracing.
Black '97 Tbird Limited Edition, '02 4R70W, 255 walbro, PST DS, PBR Brakes&SS lines, Toicko Blues & Springs, GrogTune.
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