* Wheelbase: Pickups and SUVs (like the Jimmy) have different wheelbases. The rear end width and axle length are directly related to the wheelbase, making it a major incompatibility issue.
* Suspension: The suspension systems might be different. A pickup truck rear end is likely to be a solid axle, but the details of mounting points and spring/shock placement will almost certainly differ.
* Brakes: Brake systems evolved over those years, and the 1984 and 1988 models likely have different brake configurations.
* Differential: The differential gear ratio might be different. While you *could* potentially swap the differential gears, it's added complexity.
In short, while some parts *might* be interchangeable, a direct bolt-in swap is highly improbable. You'd likely need significant modifications, potentially making the project not worthwhile. It would be much easier and more cost-effective to source a rear end specifically designed for a 1988 GMC Jimmy 1500.