My 80 series cruiser sways

[Snipedoggy]

So my family bought 91 land cruiser hdj81 diesel for towing it came with 315s (worst tires for towing) so anyways (this is year and a half later but we just ignored it) our cruiser flys all over the road as soon as you let go of the wheel it starts going sideways, also when you reach 60kmh it switche gears and it's gutless it's just dead pretty sure it's because it's the stock gears. So anyways is the problem the 315s or is something wrong like the ball joints. We are definitely going to get smaller tires because the tranny leaks if over heated.( also our tranny when it's cold it doesn't like shifting for a couple kmh) we are planning on doing a trani flush soon hopefully that fixes it (we were towing a boat up a very very steep and long hill going 110 .