Wanted to update everyone on this. I ended up just putting a few layers of tape over the existing crud and printing what I needed to this last week. It sucked and there were issues with the tape adhering properly over some areas of the crud, but I made it through.
Today I talked to Kevin at River City Graphics about it and he gave me a sample of the stuff you see below. Not sure honestly what it is exactly, but it worked instantly and the pallets are pretty close to completely free of crud now, though there are still some spots that just aren't coming off.
I mentioned in my original post about how the adhesive seemed to have bonded to the top layer of the rubber and the second picture is an extreme closeup of what I was talking about when I would try to rub it off. This area had been cleaned with the Rapid Remover stuff, but the adhesive still would not come off unless I rubbed with my thumb for a while, which led to the top layer rubbing off as you see in the photo. If I rub a random spot that does not have adhesive with my thumb, I can't get the layer to come off like this. It only happens where the adhesive has basically bonded to the rubber. This was happening before I even tried any solvents so it must have been some kind of contamination when I applied the tape originally. The only thing I can think of that I have used in the past on any tough areas is either a clorox wipe or a paper towel with 701, and neither has caused this issue in the past. Has anyone else every seen this? It strikes me as weird...
These pallets were supplied new with my Kruzer from GSG a year and a half ago, and are from Action. (GSG upgraded the size and I didn't realize until months later that they used Action pallets instead of M&R for the upgrade)
There is an interesting read on the R-Tape website as to what is happening to the glue when it gets hot. Here is the link
http://www.rtape.com/blog-post/removing-rock-hard-tape-adhesive-1
Interesting read, when you think about it adhesive , any adhesive, may be a something like unvulcanized rubber. Heat along with an additive or two is the primary element that turns unvulcanized rubber into your car tire...try dissolving your car tire....makes sense
mooseman