Driveshaft U-Joints if its a repetitive sound/squeak/grind that occurs about 4x as quickly as your wheels are turning. They should be very easy to move by hand with the driveshaft unbolted from the diff input shaft.
Except the U-Joints aren't serviceable.
I've replaced two driveshafts in different cars with good used driveshafts because of this. The first was my '94. The second driveshaft had completely failed in a car that I bought. The guy was selling the car for $1200 because he didn't know what was wrong. After starting the car and running it through the gears, I told him I thought the driveshaft had failed. I looked under the car real quick, saw my evidence and convinced him to help me push it forward. The moved car revealed the front "pinion" of the driveshaft laying on the ground. I parted with $1200 for the car, taking a gamble on the condition of the transmission tail (I've seen them grenaded after a front U-Joint failed). I put a used driveshaft in it and that car still drives like a champ today. Blake@isellmiataparts.com
might be able to get you a good used driveshaft for under $100. Alternatively FM sells new driveshafts with serviceable U-Joints for something north of $300.
If a U-Joint fails, there will most likely not be a catastrophic "ground strike", as the PPF will hold the fallen driveshaft against something and off of the ground, I believe it's the exhaust, but it might just pin it to the side of the transmission tunnel. A failure of the front U-Joint though, as I've said, can have catastrophic consequences to the tail of the transmission. This transmission came from a different car after a failed front U-Joint.
For the record, when my front U-Joint started going out on the '94, I was convinced it was the rear U-Joint by the sound. Once I had the driveshaft removed, I found that the rear U-Joint was about as smooth and easy to manipulate as a new joint, but the front joint seemed damn near welded in place. Best bet is to have someone listen from outside your car while you drive around as slow as you can go while still creating the sound.
You mentioned play in the input shaft of the differential, but does it turn the wheels smoothly by hand?