Two issues, may or may not be related.
1992 Firebird, 305 TBI Auto

This problem is getting worse and worse, when I shift from
park or reverse into Drive or OD the car jerks forward. I thought
it was a U joint or brakes. I installed an aluminum LS1 shaft
(Thus replacing the U joint at the same time). I also did the
rotors and pads (front) because they were pretty bad too.

The problem persists.
I've been told it may be a transmission issue.

Second issue is that the car shakes like hell (about half the time)
in D or OD when I pull to a stop. The only way to get it to stop is
to drop it into N or P. My idle is about 650-700, which seems
normal to me.

Please say it's something cheap like a tranny mount :hail:

Transmission fluid is full and is still red.
Was serviced before I bought the car (about 14 months ago)

 

well, jack her up and check the tranny mount. Does the fluid have any metal in it? i noticed with my last tranny (before it was rebuilt) that when I pulled the stick out to look, there was some metal flakes on the dipstick. Give that a looksie. If thats not the case, maybe the shuddering caused my the drive shaft somehow? that would make sense to me, but I am by no means a mechanic

cheeers! hope it gets to working right for ya!

oh yah forgot, my car jerks into gear sorta like yours if I don't let the car warm up, ihave asked a tranny shop about that they say its cause of the servo that was put in and some other kits in it, and from what I think, since its not warm, your idling higher, so when you drop it into gear, it goes into it a little harder cause there is more poewr going through it. Try letting the car warm up for a little bit before taking off and see if that helps that problem any. If I warm it up, i can't feel the shift at all.