It sounds like TC shudder. And as far as I know until the trans. reaches a certain temp it will not shift into OD, so that part sounds normal besides the shudder.
When was the last time the fluid and filter were changed? What kind of color does the fluid have now? Nice and red, or brown/black? Wipe it on a white towel to check, don't just look at the stick.
And yes, even if you loose OD completely due to the 4th clutch hub stripping out (which will happen eventually), you can still drive it like that. I drove my 00 GPGT like that for over a year before I replaced the trans. Only downside is more heat in the trans (due to turning more RPMs on the highway- was running at about 2K at 60mph) and less gas mileage. Once it wouldn't shift into OD at all, I just left the shifter in 3. No sense having it try shifting into OD if it can't.
That motor will make it to 200K and beyond, but these trans. are not known for their longevity, hence the current JY tranny in it now.
