help..auto trans

i own a 95 3.8 automatic trans mustang with 87,xxx miles.

the trans is not shifting out of 1st until about 3000rpm with normal driving. when under heavier load, the trans wont shift till about 4250.

also the o/d off light will come on sometimes for no reason...

trans fluid is pink and doesn't look dirty or smell burnt.

i didn't check the fluid level yet, b/c i was in a hurry this morning...

i'm new to the ford world, how does the trans decide when to shift??
based on vacuum or a computer, and if a computer is it seperate or the same as the ecm?

any help would be good...

what is the flluid capcity of this trans?
how hard is a tranny swap?