I have a nasty vibration going on at say 75mph or so. The frontend has been aligned and the tires balanced. How do I know if it's my driveshaft that's out of balance? What type of a shop will remove, balance and reinstall the driveshaft? Thanks.

Stock Steel Dr shafts are known for this type of vibration and you might be wasting your money on trying to balance it. This shaft was not designed for high speed driving. I replaced mine with a
1LE Aluminum one and the vibration is gone.

Yes you can buy a new aluminum one and consider it a nice mod.

Quicker acceleration
No vibration = SAFER
A few pounds lighter than steel one

I wouldn;t waste money balancing your current one - it should start vibrating at around 90mph... 75 seems low to me mine always acts up around 95mph and makes a big fuss when I let off the pedal until it drops to around 85ish... Youll like the aluminum, it'll be worth the money. About 300 bucks.

How do I know for sure it's the driveshaft? Is there a way to cancel out the other posibilities?

You can take a peek at your spider and how it connects to your drive shaft....on the rearend

The spider is the cross looking thing.

When you install the drive shaft, the bearings should seat equally on both sides...there is a ring around each bearing that goes just slightly inside where you secure it, install the drive shaft tranny side first, seat the spider where it should be and set your clamps with the 11mm bolts on it, tighten as needed.

I just replaced my clutch today and didnt seat my shaft properly...had vibes at 25mph!....look for worn spider gear too

-blain