Can a 98 f150 4x4 4.6 4R70W fit in a 2000 f150 4x4 5.4 4R70W
They both are a TR U.
I want to know if they can and what do I have to do to swap the 98 f150 4x4 4.6 TR U 4R70W
and to be put into the 2000 F150 4x4 5.4 TR U 4R70W?

I heard just the speed sensor.

 

The early year may have different plug configurations and speed sensor hardware as opposed to the 2000 with electronic dash.
Otherwise it will bolt up.
As the model years progressed and changes were made the trans control harness and plugs were changed were also changed as wel as the speed detection method..
Look a the plugs on both and locations of the speed sensor (VSS) to see before you buy.
They can be converted if your good at it and have all the information to work from.
Good luck.