I just bought a 1998 ford f-250 and replaced the thermostat, now the truck is sluggish and has no power. What did I do?

Check for anything , wires, vacuum hoses, anything that you might have accidentally knocked off or broke while doing the thermostat work. A good inspection. These things happen during even a simple thermostat replace. Like stupid me I checked the oil in my car and slammed the hood on my glasses, I took them off (shades) to see and laid them on the front crossmember yesterday. They did not survive, My point being things happen you don't notice at the time, a new thermostat will not cause a loss of power. Gotta be something else.

I've checked everything all wires and hoses. Is there some other way of checking to see if there is a vacuum leak?

spray carb cleaner on areas of the motor and listen for the idle to go up. that's 1 way... hope it helps

back of the intake in a common vacuum leak problem for the 5.4 it get hot a dryrotts I would look there first..its like 40.00 at the dealership.