TF 904 Auto slipping 2nd to 3rd when manual shifting @ higher revs

It is not really a "kick down" it is more of a line pressure adjustment based on where the throttle is.

At light throttle you do not need as much pressure while shifting so it has less. The shifts are relitivly soft.

At more throttle you need more pressure to have the shift feel about the same.

At WOT you need max pressure.

What you are doing is using the minimum pressure at WOT and as such slipping the clutches and or bands during the shift.

That's why folks are saying you will cook your transmission.

On Ford C4 for instance there is a vacume valve that (IIRC) does the same controls, and a real kick down linkage that actually shifts the trans down one gear under the correct conditions.