No Overdrive Cadillac Deville?

[Cory]

I have a 2000 Cadillac deville dts. My problem is that it will not shift into 4 th gear. When I'm traveling down the highway I'm pushing about 3500 rpms at 75 mph. I've had the tranny looked at a few times now to no avail. No codes at all. I recently had the tranny flushed as well thinking maybe that could be the problem, the fluid was a very dark color, but this did not solve the problem. I've also had a cylinoid replaced but can't remember which on it was, I'm assuming it was the 3-4 gear one. No one can give me a straight answer or a solution at the shops so I'm looking for possible repairs that would fix this. I don't want to bring it in and have the tranny dropped if its something else. I've heard of maybe the torque converter or something could cause this but I'm not sure on that info. The car has just 89000 miles on it and is beautiful so im hoping its something minor. If u need more info pls ask I will post asap. Thanks ahead of time for your answers, it is much appreciated.-Cory