If the rear is out of the same generation and wheelbase truck, buy the driveshaft as well if available.

 

The 9.75 axle has a larger ring gear than the 8.8 and the pinion gear is farther from the centerline of the axles and if the pinion is not shorter then the pinion flange will be closer to the trans and require a shorter driveshaft. So getting the driveshaft out of the donor truck might not work anyway.

 

I mentioned this in a comment earlier, getting a driveshaft shortened isn't very costly if you can find a place near you that can do it.
I've shortened a couple myself for demolition derby vehicles, while they held together fine, they weren't balanced which would cause problems on a street vehicle.

 

Just wanted to thank everyone for the great advice. I did decide to go pick it up. It did bolt on with absolutely no problems or adjustments. As many of you mentioned it would probably require a shorter drive shaft.. which it did. But the drive shaft was conveniently adjustable or compressible, and just shrunk... shrank??? to the length I needed.

So in short, the 8.8 differential rear end was replaced with a 9.75 from an F150 of the same year, with absolutely no problems.

Thanks again.

 

That's excellent! I do have one concern (question?) I'm not totally keen on Ford transmissions, I remember that some GM trans were sensitive to how far the driveshaft went in the tail shaft (of the trans). What I mean is, if there wasn't enough of a gap it would slide in too far during suspension travel and wreck the seal causing a leak. Maybe one of the other guys might be able to give us an answer to set my mind at ease