Have you owned this car long? and Has the trans fluid been changed recently? - i.e. per interval recommendation on this forum
I would think if you had a vacuum leak, all shifts would be harsh. Are you saying the 3-4 shift is hard? If 2-3 shift is a tooth jerker, I am not positive if this trans has one (it should), but a broken 2-3 shift accumulator spring would cause a hard shift/clunk - all the time. A cold flare may be a separate B2 concern, though. When accelerating through until a 2-3 shift, or any shift, and then quickly letting off the go pedal will cause a hard shift/clunk in most all transmissions, which I consider a normal characteristic. My 2003 Econoline, 1999 Suburban, 1985 300SD and countless customer cars all do the same thing under these conditions. Hard acceleration increases line pressure to apply clutches/bands harder to keep from slipping under heavy engine loads. A sudden decrease in throttle pressure will allow the trans to shift, but remember that pressure inside the hydraulic circuitry of the trans is still high and bleeds of that circuit slower than you letting off the pedal at the time it makes a shift. Among other things, a shift accumulator dampens the pressure of that shift for ride quality by smoothing shifts (fewer cases of whiplash
) But, they can only do so much. If you keep accelerating through to 3rd gear and let off easier, it may then shift to 4th with less of a jerk. It all has to do with when you let off and shift points of the trans. at the time you take your foot off the pedal.