99 GMC JIMMY. STEERING LOCKS DOWN SOMETIMES AFTER 1/4 TURN OF STEERING WHEEL. CHANGED PS PUMP, CHANGED STEERING GEAR BOX, CHANGED DRAG LINK, CHANGED BALL JOINTS, NO SIGN OF PHYSICAL BINDING WHAT IS WRONG?

The fuse block is located to the left of the steering column about 1/2 way down the kick panel, if you look uo, you will see it.

The easy answer is you have installed the wrong gear box, or somehow you managed to install it upside down....

Check the steering intermediate shaft connection to the steering gear box, its slip shaft and the connection to the stering collumn. if those are ok, its probably slop in the steering gear box. Some of that can be adjusted out on the gear box by your mechanic. You will see either a flat head cut or hex head inset with a lock nut on the top of the gear box. Loosening hte lock nut and tightening the bolt to factory torque setting then releasing it the appropriate specified degress will reset the gear box back to factory settings. Beware if you set this wrong you will cause your steering to bind when turning. Therefore only let a qualified mechanic do this for you.

Idler arm and or pitman arm most common.could also be excessive play in steering gear box.Tie rods/center link but usually its my first 2 choices go bad bothe of them idler and pitman common wear parts.

Welcome to the wonderful world or old-school steering systems. What you mention is very common in older GM vehicles. The gearbox gets sloppy, and with the additional issue of the drag link and idler arm, you eventually get what we old-timers call "mystery steering". It reminds you of the old movies where they have a person in the car driving with the scenery projected behind them and they are constantly moving the wheel left and right. There may be something that is really needing repair, but more than likely, you are spoiled by later model vehicles with power rack-and -pinion steering. These are very tight compared to the older vehicles and when you change from one to the other, the difference is more noticeable than ever.,

it sounds like a neutral safety switch try wiggling on the steering wheel or the gear shifter while trying to start it

Pull the steering wheel alittle hard til you can feel it move from stop to stop. Hold it in the center of that little bit of feeplay and then turn the key. It should unlock the wheel.

i am sure you have checked the fluid level in the power steering pump, check to make sure your belt is tight, if you have a single belt, make sure it is not past the wear point and letting your pump slip. find your idler pully and on the front you will see a big nut, a spring, and a pointer (kinda small and square). if it has reached the end of its movement, your belt is warn. if all above are ok, then you may have a bad pump.

check fuse and relay to switch if power to switch retest switch new doesnt mean not defective if no power check connections and wiring for damage or breaks

Have you tried to turn the stearing wheel side to side while turning the key? Sometimes the wheel gets bumped and it locks with the key in the ignition and the added tension won't alow you to turn key unless you turn the wheel from one side to the other.