Bad noise/vibration (did search)

[Tiamat-red]

Ok I recently purchased a 92 LS-L with 128000 on the clock. It was exhibiting this behavior when I bought it but for some reason I thought it was the front axles. I just replaced both and it's no better.
When accelerating it makes a loud grinding/thumping/knocking vibration from the front end. Only when accelerating.
Sort of like someone is quickly beating the transmission with a mallet. Actually now that I think about it, it sounds/feels like a deisel engine. Only in the drivetrain.
When idiling I can hear a much quieter knocking from somewhere that may be connected. I only say this because it's sort of the same frequency. And when coasting at speed it's gone.
I did some searches and am thinking front differential? I just changed the front diff fluid and it was black and thick but no little metal goodies in there. The tranny fluid was(allegedly) changed a few thousand miles ago and it does look fine.
I'm not thinking flexplate as the sounds described in the searches are more of a scraping whirring noise. And this definatly feels gear related.
I also don't think it's the engine(but have no real idea) because it's only when accelerating, and it just "feels" like it's in the drivetrain. Shakes the whole car etc.
I have most of a parts car including the engine and tranny, and was planning on building(level ten) the tranny this summer. Then swapping it in this fall. But this is too soon for that.
So I'm actually sort of hoping it's the differential.
Whatever it is I've driven the car about 200 miles and it hasn't gotten any worse. And I assume it was there long before I got it. So is it safe to continue driving it? I do love this car.

[CDG]

I would start at the easiest and cheapest fix.
Check the rear trany mount, it is a rubber supported mount with a steel pin in the middle, if the rubber lets go, the steel pin will produce vibration and sounds.
Then check the crank pulley.

Then you can start with trany related items, like from diff, and the trany itself. Could even be an engine mount causing exhaust or other contact.

These are the wish list items, hoping its not the trany, good luck.

[Tiamat-red]

Ok, now I have another question. When I replaced my left front axle today the inner splines came out with it. The part that the axle slides onto and the pin goes through. I noticed a groove on the inner end(the part in the transmission) that looked like some kind of clip should be there. I didn't give it much thought, being in a hurry to get the new axle in.
But now I wonder if this is a problem. Remembering that I had an extra "spline thingy" from the parts car when I replaced the right axle I looked and sure enough there is a clip on it. I checked the right side(which I replaced last week) and it is firmly secured into the tranny. So I put the clip on the left side spline but I can't get it back into the tranny. Is this possible with the tranny in the car? I assume it is since I have the clip out of the parts car and that tranny is still intact. Should I try using my prescision adjustment hammer?
I jacked up the car to check the rear tranny mount as suggested.
This is when I noticed that the inner cv joint was about an inch and a half away from the tranny. And started wondering if the spline should be secured, which I guess it is.
But how do I get it in there???

Edit: Ok I did a search and replace the word "spline" with "stub shaft".
But unfortunately I couldn't find out to get them back in.