Thank you in advance for any and all help,
I have a 99 150 scab that shudders around 40 mph and shakes at 65 mph( afraid to go any faster, she might fall apart ). the shudder is alot worse when I let off the gas. I replaced my u-joints and got new tires mounted and balanced (so they said) nothing changed the shudder is getting worse, I checked all the old threads but couldnt find one that mentioned it getting worse when decelerating. Thanks, confused in WA:o

 

Jack up both rear wheels start engine put truck in gear and spin rear wheels until vibration occurs. Since it is worse during letoff it most likey is in the drivetrain. Pinpoint from there. If the vibe is not present it may be in the front. Safety is very important, use jack stands

 

I did as you described (safely ) And the vibe would start around 45mph, no matter what gear I had it in. Should I have the tires rebalanced or skip that and have the driveline balanced? thanks for the help bubba!

 

Go back to who ever balanced the rear tires and have them check the balance (least expensive) You may also have a tire "out of round, or a flat spot in the tire" When you changed the U joints are you sure they were installed correctly. The drive shaft and each extension is supposed to be installed in the same position they were in from the factory. if they were not marked then you have a 50/50 chance of getting each correct. It could also be a worn bushing in the tranny tail shaft, check for play on the driveshaft where it enters the tranny tailshaft. As a last resort have the drive shaft balanced, Usually if a shaft is correct from the factory then it should never go out of balance unless it's damaged or a weight falls off or U joint problems. Did this problem start all at once or a gradual increase, is it worse on one side or same on both?? Keep us posted.

 

Bubba, there were no markings on the yoke or the shaft so I can't be sure I installed it to factory specs, I'm gonna have the tire place check tires and rebalance.Good call on the worn bushing, there is quite a bit of play in the tail. Thanks again bubba!!