Did you check rockauto? That WPM is the same PN, but half the price. https://www.rockauto.com/en/catalog...id+v8,1061514,wiper+&+washer,wiper+motor,8824
FWIW I am not a fan of the way rockauto does shipping, so pay attention if you use them. For "high dollar" single parts like this where they seem to be half the price of the local chain stores, usually whatever they charge for shipping is tolerable. But if you add parts (wiper blades, etc), invariably they start saying parts come from different locations, which adds another shipping charge for each warehouse. Its borderline unethical to me, but it's well known, and they can do shipping how they please I suppose. Not meant to be a rant, just a heads up to anyone who hasn't used them before for multiple parts. The great prices, especially on lower cost parts that you would bundle together, quickly become worse than local prices, as shipping is sometimes more than the cost of the item coming from a specific warehouse.
I'm aware of no difference on the wiper motors except the early version with the washer pump integrated with the wiper motor assembly, and the later version where the pump was moved to the washer reservoir and is thus a separate part.
The changeover was around '84. If for an '88 K5, the pump will be in the washer fluid reservoir.