07 Yukon Shakes in reverse (mostly)

[mentalattica]

It's hard to describe but my truck shakes when I put it into reverse, it's at it's worst before the engine reaches operating temp. If I put it in neutral, it's as smooth as butter. In park I can feel a little stumble/shudder every few seconds but it's really pronounced in reverse. There are no drivability issues, no power loss. It feels like a misfire if I had to describe it simply, but I don't think it's missing. No codes either.

It's been happening for months now and it drives me crazy.
So far this is what I've done:

*Plugs & Wires (oem)
*I've been through a couple cans of techron and sea foam
*Pulled the throttle body off and cleaned it real well (the back of the plate was covered in oil, is that even normal???)
*Oil Change, I use Mobil1 syn
*Trans fluid change
*Swapped back to stock intake (had Spectre short ram)
*Cleaned MAF
*Checked for vac leaks, can't find any.
*Took it to dealer and they couldn't reproduce (it didn't sit long enough in my opinion to cool back off)

The most recent repair was a new brake booster and master cylinder, but it was happening before that.

Tried to clean IAC and EGR but I can't find them, do 2007+ 5.3s have them?
I'm at a loss now. The only other thing I can think of is mounts but it really doesn't feel like a bad mount. There's not much play in the engine at all, no clunking at all either. I think it's engine related because after it reaches normal operating temps it's hardly noticeable. Any ideas anyone?

It's a 2007 Yukon SLT 2wd 58k mi. all stock as of now.

 

[InTruckDesign]

I know the trans fluid was changed, but have you checked its level, both cold and hot?

 

[mentalattica]

Never thought to check it cold. I'll definitely do so. The pan doesn't have any leaks as far as I could tell when I was under it. Where else could one lose fluid?

 

[InTruckDesign]

You could lose fluid into the transfer case if the seal between the 2 is bad

 

[mentalattica]

It was slightly below the cold mark. I rechecked it warm too. At 153 degrees (according to the DIC) it was a little below the first hot hole. I added half a quart, and got it up to the first hole. I'm going to go a day and see if I need to add the rest. Guess I'll have to recheck the pan to be 100% sure, maybe retorque the bolts.

It's still vibrating on and off in reverse but not really in drive, if it were a mount that would indicate driver's side right?