Bad Vibration while accelerating

I don't know how you came up with that really difficult diagnosis, but you're right, if that vibration is in the steering wheel. I never had a GM inner joint apart but their design is the same as Chrysler joints and I ran into a half dozen of them over ten years at the dealership in the '90s. One thing you can try is shifting the engine / transmission to one side a little. One engine mount sets its sideways position, the rest just hold it up. Shifting the engine will make the rollers run back and forth in a different area inside the inner housings. That should change the feel of the vibration. On GM cars only the cross member can be shifted sideways too which has the same effect but don't do that. That causes a common problem of altering "steering axis inclination" from side-to-side resulting in a very miserable car to drive. You need an alignment to fix that and since it's not one of the primary angles, it is almost always overlooked by the mechanic unless he is told to check it or has reason to do so.

The next thing to look at, at least in my Chrysler experience, is to jack one front tire off the ground, THEN loosen the axle nut, push in hard on the outer joint, then see if it springs back hard under spring pressure. If it doesn't the housing is probably already damaged but those springs were available from the dealer for $3.00. Never allow any vehicle weight to sit on the wheel bearing when that nut is not torqued to specs. That is typically around 180 foot pounds, but some GM vehicles call for as much as 240 foot pounds. Use a click-type torque wrench to be sure it's tightened correctly. If it is loose with weight on it, it will instantly become noisy.

The final assessment requires pulling the inner joint apart. Wipe the grease from inside the housing, then run your finger over the six highly-polished bearing rolling surfaces. If you can feel the slightest irregularity or worn spot you really have junk. If you don't feel anything, wash the housing, then shine a light on those surfaces and look at the reflection, similar to looking at the body work of a car reflecting the ground at a car show. If you see a slight wave in the reflection, replace the housing.

CV joint housings typically have to come from the dealer and they will cost more than buying a complete remanufactured half shaft. You run the risk of getting a shaft with a worn inner housing that got overlooked but that wasn't a common problem on your car model. You may find shafts in a local salvage yard too.

The engine mounts won't cause a vibration. All they can do if one collapses is allow one end of the drive train to sit lower and that will change the angles the joints go through and their effective length by just fractions of an inch.

Those wear spots in the inner housings are too small to feel but that is enough that when under load, the rollers bind as they roll back and forth twice per axle revolution. That binding inhibits their free movement so instead the shafts tug and push on the wheel bearing / spindle and those are bolted to the lower control arm which is mounted on rubber bushings. Since the spindle can be forced to move back and forth, it tugs on the steering linkage which is why you feel it in the steering wheel.

In the dozen or so worn housings I've run into, I only found one left one to be bad. All the rest were on the passenger side. If you're going to guess which one is making the vibration, start with the passenger side.

Will be out of town until Sunday night. Will check back then to see how you're doing.