My truck is loosing power when its in drive. What could cause this?

Sir: we need more information for us to help you, is the engine overheating or misfiring ? The. 6.0 Ford Diesel can be very powerful and can be made reliable, But it has a list of common failures: egr cooler, oil cooler, head gaskets, High pressure oil pump, & fuel injector failures are common, we do a full diagnostic Before making any decisions, in order to minimize surprises, and give our customer All the information we can so he can make an informed decision about a pending repair . Please let us know if we can be of service !