Drag links are not adjustable unless it has been modified.
To center the gear box, unhook the drag link from the pitman arm or steering arm, doesn't matter which one.
Put a ring of masking tape around the steering column right at the steering wheel base, then but a ring around the steering wheel, this is so you can make small marks for center adjustment.
Turn the wheel to the far right, count number of turns to the far left. Pay no attention to the spokes of the steering wheel, you are simply counting turns.
Return the steering wheel 1/2 the amount of full turns. Make a mark on the masking tape. Return the wheel to the right, and double check that 1/2 of the total turns gets you back to your center mark.
If the steering box is adjusted correctly, you should feel a slight drag as you cross your center mark. If you do not feel the drag, now is a really good time to adjust it.
Follow this GM guide for adjusting the box,
https://www.flickr.com/photos/advance-design_parts_co/sets/72157632210242370/ All GM boxes are about the same design so the adjusting procedures are the same.
Once you are satisfied you have found the center of the gear box, remove the steering wheel and center it so the spokes are in the correct location. It may already be centered and you need not do anything with it.
Once the steering wheel and box are centered, reattach the drag link. If the steering wheel is still offset, you have mismatched parts or loose parts.
1) Spring bushings can cause the axle to shift, thus making the drag link the incorrect length.
2) Leaf spring center pins can shift or break, causing the drag link to be incorrect.
3) Steering stops, can be adjusted incorrectly.
4) Incorrect pitman arm, but unlikely.
5) Incorrect steering arm, again unlikely but you never know.
6) Measure the wheel base ( front to back )on both sides, are they the same? They better be.
If your gear box is not leaking or binding, a simple clean up and adjustment is probably all it needs. Follow the above guide and do it yourself. The hardest part is getting it out of the truck. Check with the vender links on this web site for parts.
You will be glad you did it yourself when you are finished and will safe a bunch of money in the process. For gear oil, John Deere Corn head grease is the cheapest option going. You are looking for a NLGI grade 00 or 0 grease, anything else is to stiff.
Joe