Transmission Problem

[loon1wpg]

Hi all:

I have a 1999 Yukon SLE with a 5.7l v8 and a 3 speed auto trans with overdrive. When I kick it down into passing gear at highway speed, the engine revs up, bit no increase in speed, unless I back off on the accelerator, get out of passing gear and then proceed. Otherwise transmission seems to run through the gears well, no hesitations, hard shifts, all seems well. The fluid level is up to required, is fresh looking, no burnt smell or colour. The vehicle has 151K kms on it.

 

Any quick guesses as to what might be up.