91 Lumina when it shifts to Drive it acts like it is in Nuetral. I have changed the Filter and Oil. No trash in the pan. Old oil was clean. Shifts 1st, 2nd just fine. No problem with Reverse. Once you slow down car will shift back to 2nd with no problem.

I'd check around for a used tranny

Does it do that only when its cold?

Transmission fails to shift into drive when cold or hot. I took it by a local Transmission shop with no luck. They say it has to come out. I was hopping for something simple modulator or somthing along that line.

i had a 95 monte carlo, it had the 4t60e and i would put it in drive, and it wouldn't go.. like you said it was like it was in neutral... check the filter for some plastic pieces... gm trannys are famous for the thrust washer to go out of them.. its a little plastic washer near the sun gear.. once it goes the tranny will literally eat its self...