On the slow acceleration does it shift properly? If it is winding to higher rpms under full acceleration like you would expect but still accelerates slow that would point to a couple of scenarios. If it is not accelerating because it is shifting too low under full throttle that is another set of scenarios.
Before you look too deep for solutions change both fuel filters (in line and spin on). A lot of seemingly complex problems are solved with this $10 and fifteen minute fix. Dont just take the prior owners word for it that it was done recently.

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84 300SD 274K
38K miles on flatplate heat exchanger and various diesel/veg blends. prior to that 4K miles on unheated veggie blends with kero and DinoD.