greetings and thanks for any help. Just bought 97 f450 with power stroke diesel - bucket truck. from zero to about 30 mph all ok - when start getting to 35 start to feel vibration in what seems to be the back of the truck. 35 to 45 mph is bad - not terrible but bad enough. at 55 mph much smoother and at 70 purrs like kitten - fine. at 45 mph when bad vibration i put it into neutral and coast and the vibration is still there - i assume that means engine and tranny not cause so i assume drive shaft?? or something in rear ?? really dont think tires or rear as assume it would happen all the time not just at certain speeds - any advice on how to test or what simple things to try before taking it to mechanic would be appreciated - thanks for any help

 

Are the u-joints in fairly good shape? That's one possibility. If someone did replace the u-joints and didn't mark the yoke and driveshaft for alignment that would cause the problem too.
I know I have heard of jacking the truck up and putting it on jack stands. Then have an assistant start the truck and put it in gear. Have them steadily increase the gas until the odometer reads in the 30 and up range. If you suspect driveshaft then it will not be smooth in relation as it is spinning at higher speeds but then will balance itself out. There is a procedure that involves taking a piece of chalk and checking the imbalnce of it.

Maybe someone else will chime in with an easier or completely different way of diagnosing the vibration.