How to tell if 700r4 has lockup converter (and what does that even mean..?)

[benbloom]

Hey guys, im just looking into buying a truck with a 700r4 / 350 combo, the thing is, the owner says that it is not a lockup converter running currently in the trans. firstly, what is the big deal with lockup/not lockup, i understand that it lowers rpms at freeway speeds, but is this really such a big deal? what if i just drive at 65 instead of 70, is that effectively the same as having a lockup? let me know if you guys would install lockup if it was your truck, the current owner claims its unnecessary...but i would hate to buy a truck with a fried trans or fry it myself.
thanks!