Marine: Boating, Sailing, PWC, GPS and Navigation - Severe Driveline Vibration - 27' Bayliner

(Deep Breath) Ok, here goes. '78 27' Bayliner. Chevy 350/Volvo 280? outdrive. Cruising in 45' of water at about 1400 rpm smooth as silk. All the sudden, the whole boat's vibrating. Severe driveline vibration. Back it down to idle, check outdrive (figure caught a ski line or something). Nothing. Everything looks peachy. Limp back to marina, driving boat onto trailer, goose it once and BAM !!! U-Joints blow apart. Very Violent. Order new U-Joints, Yokes, all new Bellows, etc. ($716.00) Installed different Prop. Work till 3:00 AM Fri to get it together for Sat. Done. GREAT! Sat AM, a little sleepy, but GREAT! BACK IN BUSINESS!! Back boat in water, throttle up, SAME driveline vibration. EXACTLY. Bummer. Putter around at idle all weekend anyway. Caught 1 fish. Bummer. Trailer the boat, go home. Hook up the ears, throttle up, smooth as silk. Vibration only under load. Don't know where to start ! Motor ? Outdrive ? Little green men hanging on the flywheel unevenly spaced ? Would have said I must've bent a shaft when the U-Joints went, but the vibration started hours before that happened. Do I pull the motor ? Outdrive ? Hair ? What would fail that suddenly ? PLEASE HELP !!!

Did you check the flywheel housing bearings? (the ones that the primary shaft passes through). If the u-joint detonation is related to the vibration, it's possible the primary shaft is wobbling enough (blown bearning) at higher rpm loads to stress the U-joints. Something which is extremely bad on the yokes and especially the upper gearcase assembly.

Here's a link that may be of interest in terms of diagram and parts (http://volvopentaparts.stemtostern.com/VolvoDiagrams/ShowAssembly.asp?ID=7201).