I just picked up a 2006 5.4 ext. cab short bed and had to take it in for service because it wouldn't shift out of 1st and just hung at 4k rpm, it didn't do this on the test drive. Anyways, I went back and they checked it out and told me it was something to do w/ the yoke on the driveshaft and they fixed it. I'm assuming this had something to do w/ a VSS but doesn't really explain why the RPM stopped short.(similar to a clogged cat)

I drove the truck home last night from the dealer and it ran normally but traffic was so bad I didn't get a chance to see how it was on passing, etc.
This morning while trying to get on the freeway I gave it about 3/4 throttle from a 20 roll and it pulled up to 4k RPM, hung there less than a second and then popped back through the exhaust. As soon as I saw it hang I rolled out of the gas and thats when it backfired. It almost seems ignition related.

I tried searching these symptoms to see if this was common or what else I should look for.
Truck fires right up, no ticks, no audible timing chain slap, firm/normal shifts when its not acting up, stock from air box to exhaust tip, stock ride height, etc.

Thanks in advace for the input.

-Carey