1993 Firebird 5.7L & Transmission Problem?

I purchased my car about a year ago and have managed to put an extra 20,000 miles on top of the 100,000 miles it came with. When I got it, it shifted real hard from Park to Drive and would kick...I lived with this for a while, a few months back I thought it was time to change the differential fluid and tranny fluid and filter, so I changed that and the spark plugs. About two weeks ago, I was a stop light, and when I hit the accelerator (normal gas) I heard this loud sound that almost sounded like something had ground real hard against something else, then the car went fine. For a while I thought I had merely slipped on some loose gravel or something. Now, when I shift from Park to Drive it still shifts hard, but when coming to a stop light the original clunk I had has now turned into two clunks, almost as if it's slipping real hard through the gears. In addition, if I come to a stop, my drive disengages: I can move to Neutral and back to Drive with no gears changing. I then have to shift to 1st gear all the way back to Drive just to get the car moving, sort of embarassing too, cause all you hear is an enginer revving, but not moving. I can say that I do hear a "clicking/grinding" sound when the engine revs while in Drive. Does anyone know what this could be? Obviously it sounds like a transmission problem, but is it necessary that I spend $1000 on a new transmission, is it worth it considering the car now has 120,000 miles, can something else be repaired, and if replacing the tranny is the only solution, can I do it myself over a weekend? Mind you, I've worked on my car a lot, but letting a mechanic steal an extra thousand dollars from me for a job which I think I can do, is a road I don't want to take. Then again, I'm hoping it's not the tranny at all :)

Sorry for the long post, just hate reading posts where the questions are extremely vague. Hope someone else benefits from any solution(s) to this problem.

hmmm check to see if you need a new tranny mount...costs 120bucks and should only take an hour or two to fix (from a professional) . I will get back to this with some more info.

Thanks for the reply! I did recently replace the tranny mount since I also suffer from the rubber brace tearing and for a while I had that "kick" from under the passenger's seat. I can't say I feel the same now, as it's merely an engine revving now while in Drive, though it still shifts fine through all gears. I'm starting to believe that it may be an issue with the shifting from 1st to 2nd? while in Drive, but I can shift just fine manually. Still worred about the loud clunks I hear when coming to a stop. Thanks again!