Compare the piston relief on the steering boxes, the J20 box (if it's still stock) has an enormous piston which gives it a lot more assist. I had one, I swapped to an S10 box and will not be going back. If you're running giant tires and trying to turn on high traction surfaces, the J20 box may be for you. If you want road manners, the S10 box wins hands down.
This is why the J20 front D44 is rated higher than the J10/wag/chero version. I can't remember where that pic came from, but it is not mine and I certainly can't claim credit for it. Great reference, though.
The J20 was a 3/4 truck, I believe the lowest GVW I've seen on those is ~6800lbs. For reference, J10s/wags/cheros were usually around the ~6200lbs mark. The J20 could be had in configurations up to 8400lbs GVW. Your vin will indicate which version your truck was from the factory. The blocks you have there are not stock, I don't believe blocks were ever stock on these. The J20 springs, even the softest ones, will likely rattle your teeth out in a wag, but they will have higher spring rates.
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1983 J-10 - 4.6L(MPFI)/CS130D/Hydroboost/NV3550/D300/44/44/3.54/Disc-Disc/32s/42 gallon 'burb tank