All the advice above is good stuff to check. If you have a 55 1st with a 283, someone may have moved the steering box to put the 283 into your truck. After moving the steering box, they would have to "fiddle" with the pitman arm to re-align it with the steering arms. If that isn't right, it could cause "exciting things" to happen. You said you had radials on the the front and some old mud grips on the rear. If the mud grips are bias ply, that would cause steering issues also, (see cmayna's reply above). The two rear bias tires would react differenty to the two front radials.
