Your trans (the 545RFE) does not have a governor solenoid, so that's not the issue. If it gets "stuck" in 3rd gear then either the PCM is commanding it to be in 3rd (so nothing is wrong with the trans), or it's in limp-in due to a trans fault (not the P0068). In limp-in, pulling it down manually to 2 or 1 will indeed still downshift the trans, so you may have a trans fault code also being set, although you should have found that when you had the codes checked. I suppose it's possible that the P0068 fault means the throttle position reading is screwed up enough to affect the transmission shifting. But if it's in 3rd gear at a dead stop, to me that would definitely indicate a trans fault code is set (or you lost the 12V power feed from trans relay to the trans).