<VV> Non-Vair, Cutlass Automatic Question

[monza67 at aol.com]

I have a 92 Olds Cutlass Supreme, 3.1liter with 4T60 Hydromatic overdrive tranny. Car has about 82k miles and the tranny started to shudder on takeoff the other day. My son checked the fluid and it was low, so he added some. Car will not move forward in drive or any other forward gears now.?It does go in reverse just fine. I drained the fluid, replaced the filter and tried it again with no change. Could it be the modulator? If so, where is it...I can't see anything that looks like a modulator to me. I'd much rather be working on my Corvair than this car, but that's the way it is sometimes. Any help would be appreciated. I'm thinking this may be an opportunity to get rid of this car, but if it is something that is relatively easy to repair, I can't see getting rid of it. 
Thanks, 
Chuck Prosise