Won't stay in overdrive

I have an "88 Blazer with a 700R4 transmission. I recently replaced the trannywith one I got from a salvage yard but I'm not sure what year Blazer it came out of. I'm not sure how long the tranny was left sitting out of the vehicle. It shifts fine but won't stay in overdrive with any kind of "load" on it. I can feel it shift into overdrive once I get up to 55 mph or over and let off the gas. The torque converter will lock also. But if I press on the gas and get the RPM's up over 2500 it shifts back into drive. Even when going down hill. It will not shift into overdrive untill I let off the gas and the RPM's drop below 2500 at 55 mph. The RPM's have to be even lower at lower speeds. It has new fluid and filter, even in the torque converter.

Anyone know what could be the problem?

Sounds normal to me.

It is supposed to unlock when you put a load on it.

It is overdrive which is meant for cruising only.

Check the adjustment of the TV cable. Back off on it a little and take it for a spin and see what happens.

The tranny will not stay in overdrive even if cruizing with very little "load" on it. It will not gain any speed once it has shifted into overdrive. It will downshift into drive if I get the RPM's over 2500. Even going downhill it will downshift to gain any speed. It will not even cruize at higher speeds. I've adjusted the TV cable manhy, many times and even unhooked it to try and see if it will stay in overdrive and not downshift. I basicly have to keep my foot off the gas in order to keep the tranny in overdrive. It will downshift at any speed. I've even got it up to 70 MPH while going down hill and barely gave the engine gas and it will downshift. It's murder on my engine when it downshifts at that speed cause the RPM's jump to about 4000 at that speed and the engine sounds like its racing. I've never had a transmission downshift so easily b4. It basicly won't stay in overdrive. It also stays in 1st a little longer than normal and will sometimes feel like it shifts into 2nd and then into drive so fast it feels like it skips 2nd all together. It just doesn't feel normal for it to downshift so easily out of overdrive.

What I mean by having a "load" is haveing any kind of pulling force on the tranny. Like trying to increase speed. Even downhill when the load force should be less than trying to pull uphill.

If anyone has any ideas as to what i could check, please let me know.