OP, I have 03 GTP, and similar issues. At about 125k, I got a progressively bad shift bang into 1st. Did a zzp shift kit, then a TEP shift kit. Then noticed a "hum" from front right. All along I have a clunk from front right/underneath when engaging gear from stop, or reverse. Did a bunch of reading, and found hum is symptom of bad diff and bad water pump. Within weeks of noticing hum, I got a LOT of right wheel spin, all the time. It was fun to chirp it at 30, but I knew it was wrong. So I replaced the diff and hum, and right wheel spin were fixed. It never leaked and the case, and everything else was still good, from what I could see. Still had shift-bang and clunk. I recently replaced the shift solenoids and shift bang is gone. I intended to change the subframe bushings too, but couldn't come up with a way/don't have the tools to get oe out and they didn't appear bad, so I left oe in. 04 is different design, but when I bought my subframe bushings I found that Dorman and Napa have fronts, but rear subframe bushings are not as readily available, got them from gmpartsgiant.com. Use the diagram. Enter your vehicle info, select from "section": body mnt'g, a/c, ..., select from "component": body mnt'g, select view info, select view. Not cheap and slow shipping, but IMO better than the stealership.

Let us know when you get it right!