just bought a 91 f150 4x4 with a straight 6 front end starts to shake at 45 mph previous owner states he put on some very used 33 on the truck..dont know if he balanced them before he mounted them...any ideas what might be wrong thanks in advance

 

A good start would be having the tires balanced.

 

You should be able to see the wheel weights if they've been balanced - but they could still be out of balance.
Jack it up and check for play in the steering or ball joints.
Always a good idea to spend $60 or so having a mechanic you trust go over an unfamiliar truck, just to make sure nothing's going to fall off and cause you to have a wreck. Money well spent.