The truck is a 1977 F-250 2wd. Long Bed reg cab, T-18 4 speed trans. It does have the split driveshaft and the yoke on the trans is bolted on, not the slip yoke style. I have a bad vibration at 2500 RPM in 4th gear. Its not tires, because if I shift into 2nd it goes away. Its also not engine related because it does not vibrate at 2500 RPM in any other gear. It feels like driveshaft vib. I have had the shaft balanced and all joints replaced including the stub on the center slip yoke. Any ideas?
It seems to be much worse when the truck is loaded, which changes driveline angles.

 

OK, now 2500 RPM in 4th. You can shift down to 2nd at that speed?

Or are you comparing 2500 in 2nd to 2500 in 4th?

Sounds speed related, which takes you to tires, and wheels. If they balanced the shafts, they would have caught any bends or dents. (ya think?)

Any funny noises back there?

 

Yes I can the truck has a high rear axle ratio. 2500 in 2nd yeilds no vibrations. It may be tires/wheel, but the frequency feels too fast to be that. The other reason I'm not looking closer at wheels/tires, is with my last set of tires the vibration was the same.

Also with the change in the vibration with the change in driveline angle, seems to lend itself to the center slip yoke. Is it possible for the yoke to wear a spot in it and then when I load the truck it pushes the yoke over that worn spot and worsens the vib? I do not think the driveline shop repaced the slip yoke, just the stub.

No noises at all, I have inspected the rear axle and transmission internally, they both look new inside. Neither have any leaks etc.... Truck is great, but the vib annoys me.

 

I think what was asked is: does it vibrate in 2nd or 3rd at the same SPEED as in 4th??

If so it sounds front end related; possibly a bent rim, bad tie rod or wheel bearing. A bad driveshaft would vibrate at most any speed.

Any wheel/tire combo can be balanced; bent rim or not. But when you drive with it it will still shimmy.