I have a 99 F250 (4.6L) that runs rough during wet conditions. It runs perfectly fine when dry, but even just road spray will cause it to misfire so badly it barely runs.

It's been to the dealer multiple times they say it only needs tuned up and can find nothing else wrong It's been tuned up twice. Immediately after the tuneup it's marginally better: road spray's no longer enough but rain or wet snow and it's back to running rough.

Does anyone have any idea what may be causing this?

Also, the rear end just started getting a vibration in it on take off. Are there any known problems for this? It doesn't have enough miles that it should be wearing out.

Thanks.