To the best of my knowledge, all the GMT400's came with was a gov lock in the rear axle, RPO code G80.
The fronts were all open.
Gov lock is meant to be a low speed "posi". Can't recall the speed, but I think it's ineffective (IE: disengaged) above 20 MPH. Lower than that both wheels should be spinning if you have a gov lock rear axle.
Gov locks are not especially robust units. Many have blown them up by spinning one wheel in gravel and one on pavement. When the gravel wheel grabs pavement, the little teeth that lock the differential on the swinging lever sheer off and you're back to an open diff.
Except now you have shards of metal rolling around in your rear axle, which often becomes terminal for the whole diff. That is; assuming it didn't frag your carrier right away when it gave out...
For a gov lock axle, you have to change out the entire center diff guts as none of the aftermarket lockers will work in that particular carrier. At least, none to my knowledge.
For the front, if you're got a 1/2 ton you're out of luck. There's no lockers made for the 8.25 gm IFS differential.
I believe there is one for a 2500 front diff since its a bigger gm 9.25 unit. I want to say airlocker, but don't quote me on that. Mybe it was an Eaton unit, just can't remember.....
Front lockers are not a good idea in GM ifs diffs for the most part. The center section is aluminum and is given to lots of deflection when strained too hard. Deflection is a carrier housing equals "BOOM". Lots of guys are running them in sled pulls with upwards of 1000 hp duramaxes though. but they do blow them up and it's usually quite spectacular when it goes.....