Your going to have to post results using the TB spacer with the 4.0 TB. Since I did mine I get small bump in the idle when it warms up. Almost like it is getting too much air or like its going through a pattern. You can almost count 1,2,3,4, bump then skip a cycle and then the same 1,2,3,4 bump. Maybe Jeffery or some other good soul on the board has encountered this. It does get kinda old.
I haven't had a chance to pull the tb spacer off and see how it runs without it. I changed out the front O2 sensor and made the bump less noticable but it is still there. Prior to the upgades I noticed it a little but not near as pronounced. The TB is sparkling clean because I soaked it and took off and cleaned the sensors before installing. I did reinstall the old 2.5 sensors because the used TB I got had the flat ones.
Good luck these things are damn fun. I haven't done any serious wheeling but the back roads and little mud puddles have been a blast. Later Mike