If the engine has not had regular crankcase flushes (every 15 to 30k), does doing one at >100k cause any concern for seal/gasket leaking?

 

At your mileage, having just purchased the truck, you should flush it, not with SeaFoam though but with 6 quarts of ATF.
An engine flush will not cause leaks but will remove alot of slugde and varnish as well as improve oil flow.