I had a spare driveshaft shortened to install in my F250 after I did the ZF swap.....and apparently it had some issues....
Before commenting, know that BOTH front wheels were chocked front and back, the truck was on jackstands on the rear and it had a tractor nudged up against teh front of it to prevent it from taking off. Safety first.
Could be bad U-joints, loose pinion, bad carier, etc.
I was gonna say balance issue, but I wouldn't see a balance issue at such low speeds.
IDIDieselJohn 1 year ago