I feel a clunk when tranny shifts into 4th.
 

Recently my truck has started to shift hard into 4th gear. It is a 2000 extended cab Lariat 4x4 with 4.6 auto. It only does it at higher speeds like when I'm going onto the highway. So far it seems ok at slower speeds. Any ideas? I'm hoping this is a mount or U joint issue and not a tranny problem. With 220K I am not optimistic.

Mine was a combination of a weak fuel pump and the drive shaft neck. Yes after a tranny rebuild it did the same thing. I saw a YouTube video on this problem. Remove the drive shaft, the yoke where it fits into the transfer case needs a thick coat of grease on the spines. The bump is created by the splines grabbing. Change the u-joints while you are at it. Note the rear bracket will probably break unless you use a press. I suggest taking the assembly to a drive shaft shop and have them change the joints. Then grease it up and put it on.