Slight trans slippage on highway

[Growl]

I may be wrong, but it seems a little like the tranny is slipping just slightly at cruising speeds(60mph) in 4th gear. Nothing while shifting, and just a slight slip. I wouldn't have noticed it on a bumpy road, but they just repaved.

Also, it seems to have gotten worse over the last week or so. Any ideas before the transmission shop trys to sell me a new transmission?

I'm running the Vortec 8100 in a 2500 Yukon XL (2002)

Thanks in advance.