I was reading this question, and was trying to remember an article I read many years ago. It seemed the article was indicating that there is a chance that one of the boosters from one of these missions could be going faster than the probe it launched, and thus be leaving the solar system faster. I can't find the article in question, but it got me thinking, where are the upper stages to the Voyager and Pioneer rockets?
I am infocat13 who wrote that unmannedspaceflight.com article, I base my opinion based on a personal communication with Dr. Stone, the PI of the Voyager project and many AIAA astrodynamics journal articles.
Mr. Lasher, a JPL astrodynamics specialist, informed me of the Pioneer 11 missing its B plane Jupiter aimpoint before TCM-1, so that solid motor is most likely not in a solar system escape orbit but more likely in a orbit like the Ulysses spacecraft.
Dr. Stone was kind enough in 1994 at my request to have someone run the state of the spacecraft before TCM-1 (before the spacecraft did it own first maneuver) to establish for certain the locations of Voyagers 1 and 2 third stages. The result was that one of the third stages might have entered the Jupiter B plane impact radius! This would have been for the Voyager that had the traveling wave tube malfunction, so I am unsure if we have a true vector for that Voyager. I believe that the wrong data was used and so both Voyager solid stages, like Pioneer 10, are in solar system escape orbits.
I have spent several decades now trying to convince PI's to (that would be you Dr. Stern!) preserve the historical knowledge of the process of getting human artifacts in deep space and their final disposition. Dr. Stern, like Dr. Stone before him, does a wonderful job of trying to answer questions on places like unmannedspaceflight.com.
EDIT I found an image per my personal communication with Dr. Lasher (retired JPL)
Note the off-nominal Pioneer 11/ TE-364-4 (Star motor) injection aim point
Edit 30 Aug 2015
It isn't known exactly, as we don't have a good way of tracking them. However, this question was brought up in the form unmannedspaceflight.com as @Hash mentioned, which mentions a Wikipedia article. Bottom line, we believe there are 11 artifacts that are leaving the Solar System that we launched:
5 probes (Pioneer 10, 11, Voyager 1, 2, New Horizons, and 4 booster stages (All but Pioneer 11), plus two spin weights from New Horizons. Of these, we can confirm that all of the spacecraft will leave the solar system, and only 1 of the booster stages (New Horizons), and the two spin weights. The other booster stages can't be confirmed, as we lost contact prior to the flyby that would put them on a solar system eject route.
Of the unknown 3 booster stages, we can make a few guesses. Pioneer 10 was about 132,000 km above Jupiter's atmosphere. If the booster stage passed further, then it would be further away, and closer, it could be closer. My guess is that it passed further away, which would put it on a fairly slow path leaving the solar system, but roughly in the same direction as Pioneer 10.
The Voyager 2 rocket stage should be leaving the solar system, but at a much slower pace than Voyager 2 itself. It will be headed most likely at a bit of an angle, but in general will be tending towards the direction that Voyager 2 left from Jupiter, albeit much slower. The same thing can be said of Voyager 2's upper stage as well.