Trans issue: 4L85e shift problem

[68GMCCustom]

Background:
I have a GM Performance Parts 4L85e behind an LY6 6.0L swap and using a B&M truck MegaShifter to shift it and maybe 1k miles on it. Both were brand new when installed. When I 1st installed the LY6 it was carb'd and ran a (built?) 200-4r. Well the 200-4r couldn't handle what the LY6 was putting out so I put in a 4L85e to replace a 200-4r (which the MegaShifter was bought for originally) and it shifted manually thru all gears just fine. Over time, I have lost the ability to pull it down manually to 1st...though while shifters in 3rd or Drive it will downshift to 1st just fine (from roll or stop) and 1-2 shift fine. But when I try to manually pull it down to 1st it stays in 2nd. I have adjusted the cable at the trans numerous times trying to fix it but nothing changes. With the cable set about as good as possible in Park and 1st... its slightly too long while in D (from 2nd to Reverse wont allow the pin to slide in and out, just P and 1) I tried again yesterday and didn't even get to test drive it. I setup my AeroForce gauge to display gear selected and from 2nd it wouldn't pull down into 1st now matter how I tried. Only difference since when it worked is about a thousand miles of use. One interesting side note and possible diagnostic lead...I sometimes feel like when in N and reving the motor that the truck is trying to move forward a little. Haven't verified this on flat pavement tho.

So my questions...
Theres no reason something in tuning would develop to keep the ECM/TCM from selecting 1st is there?
Or is it just my MegaShifter cable already needing replacement?
Could I somehow have damaged my brand new trans with the shifter adjustment being a bit off, without knowing it since all was fine to begin with?

Thanks for your thoughts!

If I put this in the wrong spot please move it Admins.