We installed a new transmission in a 1995 ford f-250 diesel.
We heard that since we did so we should be looking out
As the truck will start having a lot of problems soon.
Is this so?

What kind of problems?

If it was the exact replacement transmission, nothing could be further from the truth. This is the ideal time to change fluids in the transfer case and front/rear differentials. I only use full synthetics in rancher and disaster preparedness diesel trucks I service. This includes the engine, power steering, transmission, transfer case, differentials, and suspension.

Not to mention having flushed out the translission cooler and lines to remove debris left from the failure of the previous transmission before they get sucked into the rebuilt unit. ( I also assume the torque converter was replaced with the transmission as well as it should have been. Debris being the main reason..

It's a good idea to change the transmission fluid and filter early on new and rebuilt transmissions. The fluid will darken sooner from clutch pack dust and wear metal. Ford transmissions are tough and well built--I wouldn't worry too much about things going wrong, especially if you follow a good preventive maintenance program.