Weird problem with 4th gear

[jxw004]

So my 2004 tahoe 5.3l has started acting up in 4th gear. It will cruise just fine at 75 but will not hold any speed above that unless I shift it into 3rd. All other gears work fine, it is lifted on 33s but has been this way for a while and it started this a few weeks ago. The torque converter will lock (if you hold the accelerator and lightly press the brake it will unlock and jump up about 500 rpm). I am getting crappy mileage, about 11 on free way. Any ideas on what could be causing this? I have a diablo programer on the way and one of the options is to clean the TCM memory, would this help or no?

 

[SLCHOE]

Clearing TCM memory will just erase any codes that are stored and reset all adaptive memory (ie: learned shifting patterns, etc).