I just posted about this same issue. I had 37's with stock gears & the speedo read correct with EFI tuning & no issues. I went up to 40" tires & 4.56 gears, now I get the ABS & parking brake lights (also disables the parking sensors & gives "service trailer brake wiring" message).
As far as I know, the issue is that EFI adjusts the speedo signal that is read off the rear tires (in the T-case I believe) & your ABS is picked up off of the front wheels which are still set to the stock tire size (can't be changed with EFI). These two signals can be off from each other up to a certain percentage before it will trip the ABS light. This is why you can go up to 34.25" before it comes on, it keeps it within that percentage.
However I thought you could up the tire size in the ABS module with a Tech 2 to allow you to go a little bit bigger? I believe that's even what the sticky on this site states?
I tried to change the ABS setting in the EBCM with a Tech 2 but apperently it doesn't allow you to, atleast on the NBS trucks?