99 Z24 5 Sp Problem! Help plz!!

I'll start off with what i have. It's a 5 sp manual 99 z24 Coupe, has Specter cold air intake, thrush turbo muff, and maintenance. My parents bought me this car about 3 months ago. All the problems before i got the car that were fixed included:

-Front replaced (bumper to windshield)
-Engine replaced with 2000 Grand AM 2.4L
-Trans replaced twice
-And some general maintenance

The problems i now have are:

-gas gage malfunctions at about 1/4 tank
-speedo goes speratic at times
-clutch in getting worn (but not hard to shift still)
-gas hog (about 20 MPG, like an eco V8)

But my main problem i need to know asap is my new found problem. I can't tell if it's my trans, differential or rack-n-pinion. Just this morning i drove the car about 20 miles and stopped at the bank. When i left the bank, my car jerked wen after the clutch was already fully engaged in 1st. It still shifted fine but i got it up to second and it did it constantly (almost like i was running low on gas but with a dull thud noise). But then goin down the highway, about 55 mph, it started to jerk really hard to the right when i got above 25 mph. And it was still doing the jerk thing but it wasnt thudding just acting as if it was low on gas (and that's deffinately not the problem). So plz if anyone can help me today that would be really appreciated! Thanks

Trans is blown. But will accept some insight on a swap from the Isuzu (1996-1999) trans to the Getrag (2000-02) trans. Just need to know wat all i would need in order to do the swap. Any help would be helpful. Thnks

here's a how-to with a ton of pics...

http://www.j-body.org/forums/read.php?f=41&i=36216&t=36216