Tomas, I certainly hope your tires are not 15 years old. If so, don't waste your money balancing them. They'll start to delaminate in short order.
That twitchy feeling surely sounds like a tight screw. Before you go ahead and try to figure out the proper procedure, loosen the locknut and back off the allen screw about 1/8 of a turn and see if the problem is reduced. If so, then separate the pitman arm to center lick connection, and the steering coupler joint, then follow the procedure in the manual.
It might be easier with the steering gearbox on the bench, but you might be able to leave it on the frame with the hydraulic lines attached while checking the torque load.
Glenn Uettwiller
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