The '96 K3500 should be a full floating rear (10.5" ring gear) which should be evidenced by the large hub sticking through the wheel's center bore, with 8 bolts holding the axle to the hub. Your '90 K2500 probably has the semi-floater (9.5" ring gear.)
While your '90 K2500 has a 3 speed (TH400/3L80) the '96 K3500 should be a 4 speed (4L80E) if auto, or 5 speed (NV4500) if manual.
The rear end widths are the same since they're both K models, you'll get a stronger unit with the full floater, but the #1 concern is that it's the same gear ratio as your existing one. Got to match what you already have so you don't have problems in 4x4. The saddle in the pinion yoke for the U-joint is *probably* the same but don't bet money on it.
You could get the gear ratio RPO code from the sticker in the glovebox (look for any 3 digit codes starting with G - for example GT4 is 3.73, GT5 is 4.10) - but the best practice is to go ahead and pull the cover and confirm. Stamped into the edge of the ring gear will be two numbers, divide the large number by the small number. Example: 41 and 10, divide 41 (number of ring gear teeth)/10 (number of pinion gear teeth) = 4.10 ratio.
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The other benefit of pulling the cover to confirm is you can get an idea if the thing is beat or not. Check for chunks, broken teeth, lots of material stuck to the magnet..
Richard
