Q: Replaced 5r110 transmission in 2004 f350 6.0 4x4, hard shifts or slipping transmission. Also doesn't always engage reverse.

If this is a new or rebuilt transmission, and external controls (throttle position sensor, solenoids, TCM, harness, etc.) have ALL been diagnosed and tested, the transmission is likely defective and should just simply be returned to the vendor, again though ONLY once all "external possibilities" are ruled out. If you bought the unit from a vendor, they do have a technical support staff to handle basic inquiries. If you installed a used transmission, it is even more likely it is just simply defective. Transmissions rebuilding is frankly easy BUT it requires an incredible amount of caution, measurement and care in assembly. So, it is VERY time consuming, especially when you add in the research that’s necessary to procure internal parts whose DESIGN has changed. Typically, what will happen is the unit just gets thrown together, to save TIME, and then problems are predictable. To determine, one way or another, if the problem is external or internal, you can schedule a shift diagnostic and the responding certified mechanic will run tests, in particular looking at gauge hydraulic pressures in reverse, to see if the problem is internal or external. You can do that yourself, too. If it is not running in reverse, check hydraulic pressure on that circuit. If the pressure turns out to be wrong, that will conclude the diagnostic right there as pressure is "typically" only off if there’s an internal fault. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.