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Old 02-25-2010, 08:17 AM
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GM rear end info

Dealing with a 1989 GMC S15 Jimmy 4x4. I`m finding info that says the 7.5 and 7.625 rears that came in these the guts interchange between housings.
Also that they came with 26 and 28 spline axles. Is there a way to tell what mine is? It`s a open diff and before this thing goes back on the road it`s gotta have a posi. The little info I`ve found says it should be a 7.625 rear with 28 spline axles, does this sound right? Last question is info says that in 89 they changed it some how and later units from 90 and up won`t swap in the housing. I`m trying to get solid info, I hate it when it`s hodge podge and I keep finding different answers. Thanks for any help given.

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Old 02-25-2010, 08:58 AM
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According to THIS site, I'm inclined to believe that if you retreive the axle code from the housing axle tube (passenger side, forward-facing), you should be able to tell what size the diff is.

AFA everything I have, it should be a 28 spline.
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Old 02-25-2010, 09:18 AM
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Your truck was built around the time that they were being changed over from 26 to 28 spline. I've been told by other S truck owners that they have found 26 spline axles up into the '90s. I would not buy anything for it until you remove an axle and count the splines. Another option would be to buy a 28 spline posi and the change the axles if you have 26 splines. The 28 spline axles would be a good upgrade if you have 26. All 7.5 and 7.6 parts will interchange as long as you use the right combination. Be sure that you get the right series of differntial for the gear ratio that you have. S10, Late Model Camaro/Bird 7.5 Posi fit G-body rear end?
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Old 02-25-2010, 01:12 PM
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I`ve delt with the 7.5 in the past with my cutlass, Since me and 2 other friends run these cars we used to head to the junkyard and start checking 82 - 87 Z28 and Trans am rears looking for posi`s, we would yank the guts out and stick`em in our cutlass. However when it got to the 7.625 diffs is where my mind got cloudy as I never really looked into them much, I kinda thought it was a oddball since it was just a hair bigger than a 7.5. Great info yall gave me. I noticed when I looked under my jimmy the rear`s housing looked identical to a 7.5 and now I know why. I hope the current axle`s in it are 28 spline, that would indeed be a big help strength wise and should hold up behind the 300 horse 4.3 I`m building to go in front of it as I`m not real trusting of the 26 spline. Really appreciate the info guys.
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