In my regular monitoring and based on my readings, the STFT typically "oscillate" somewhat in conjunction with the o2 sensors basically as the system responds to trying to keep the expecting mixture required (14:1 or thereabouts). The LTFT is what it is, "long term" (ie. "constant"), is the "coarse" setting. Thus, the system operates at the "point" with the "correction" in and around that point... meaning somewhat "less" (negative) and somewhat "more" positive, relative to 0 not the LTFT point. So even with a significant negative LTFT (-10/-15), it is possible to have both + and - STFT response as the system still attempts to maintain the fuel air mixture ratio. It appears that your system continues to see significant "negativeness" basically moving the "sum" of the two fuel system controls towards -25% which will eventually cause either the pending code or code depending more on how close the LTFT comes to that bogey.
As for whether there is a vacuum leak or not, I would say that is not your problem as a vacuum leak would likely draw in some "unmetered air" for which the system would be compensating with MORE fueling and hence positive LTFT. Again, hopefully we are each talking the same paramters and not "cross talking" (ie. misunderstanding one character letter).
One thing related to your recent data, is the idle rpm. Perhaps the I6 has higher idle (at around 600) so perhaps the V8 is lower, but that value seems a bit low. Does the truck idle well?
Further, related to the vacuum question, the value at the PCM (which you display) is a sensor reading, which could be bad wiring and / sensor OR could be an engine issue in terms of mechanical issue (valve or otherwise). Earlier posts in the thread provided some measuring tests to determine if the MAP sensor is functioning well in terms of responding to vacuum and changes there in. Alternate test to confirm readings would be to get an external vacuum gage and take a "parallel" reading (ie. at idle and at medium constant throttle) to basically confirm the readings. Go from the result. Since the engine itself "appears" to run relatively fine (yes?), it is unlikely to be a mechanical integrity problem, hence my guess is its more to do with electrical "things" (MAP or there in).
One other question, what altitude are you at?