Have 2000 Buick Park Avenue 3.8 non supercharged. Just bought the car & it had valet key no ignition key & security light on. I was driving the car on interstate an got off exit an car was not wanting to move while in drive did it 2 times on way to work with stop & go. Got to work pulled in went to move it in drive an nothing put it in 3 an moved it. I tried after work an D 3 2 nothing. Got it to friends house changed trans fluid replaced straight pipe with cat & replaced muffler & got new ignition key programmed & replaced map sensor. The security light went off & back pressure codes went away & still only have R & 1 to move the car D 3 2 still nothing when you put it in those gears. Could it be trans shift sensor? Any ideas what might be wrong?

Could be electronics disconnected.

Could be that the transmission has some mechanical issue.

How many miles does it have on it?

It drove fine right after i got it I drove it like around 150-200 miles before this started. An got it with 282,148 on it. You can shift it in to D 3 an hear it kick into gear it will now go into 2 but hard it goes into R & 1 just fine. Dont know how long it sat before I got it an all the fluids needed changed in it. It does have a leak on the radiator nothing major. No codes coming up to cuase it to do this. Could Oxygen sensor or MAF sensor cause this? This is daily driver. Any ideas?

I'd bet the O2 and MAF are not causing this.

I got it at a transmission shop an they said the ear on the trans is broke need another trans. Going to find on at a salvage yard an get it back on the road asap.

I haven't heard of that too often on one of these. Any idea how that happened?