Step #1, have someone in the cab move the steering wheel left and right hitting resistance each direction while you are under the truck, wheels must be on the ground. Look at the tie-rod ends, drag-link ends, and steering gear box. You need to see where the actual play is coming from. A good working system will not have any side to side movement at any of the joints. The pitman shaft should move with the slightest movement of the steering wheel. Also check the leaf spring mounting bolts and shackles. Most likely it's the gear box, but you still have to check the other parts.
Step #2, if tie-rods and drag-link ends look good, jack up the front axle and check the kingpins and wheel bearings. No free play is allowed at the kingpins, and the wheel bearings should have just a very slight touch of free play. Grab the tire at top and bottom, while alternating pulling and pushing, you should only feel wheel bearing movement, and that will be very slight.
Step #3, Read this manual (
https://www.flickr.com/photos/advance-design_parts_co/sets/72157632210242370/ ) It's the best information you will find on the GM gear boxes. Your '42 will be just the same as far as adjustments and rebuilding procedures go.
Step #4, removing the box for a good cleaning and adjustment is the best way to go, but it can be done in the frame.
Step #5, after cleaning and/or adjusting, fill with a #00 or #000 grade grease, John Deere corn head grease is used a lot, CV joint grease, and riding lawn mower gear box grease. Tractor Supply has similar grease. It must flow back in on itself ( self leveling ). 50/50 #2 gun grease and gear oil will work if thats all you have.
Step #6, grease the whole suspension and steering with #2 gun grease. A drop or two of thick oil on the upper steering mast bearing doesn't hurt if you have the wheel off.
Depending on the condition of the truck, the gear box may be to worn out to adjust. The pitman shaft ( sector gear ) teeth wear in the straight ahead position and no amount of adjusting can get rid of it. Chevrolet used the same box on all the trucks except the COE, so you can use parts from any size truck. The mast lengths changed, but the internals stayed the same. The venders now sell new masts with the worm gear, and pitman arms show up on e-bay every now and then. '41 - '46 are all the same except '41 uses different size roller balls on the worm gear. You can swap the whole unit into the '42-'46. Bearings and gaskets are available.
