The Owner Information Guide for 2005 - 2009 models suggest at 30 months or 50,000 km and 60 months or 100,000 km for both transmission and trans axle service I was at 108 months and 77,000 km for my second service.

I'm at just over 4 years and 55000K so I guess I'm due?

Sounds like it.

I've read somewhere else that using better oil (I don't remember what kind), will improve the shifting or at least make it smother, ever heard that before?

Are you auto or manual? Are the sixxers different from the V8's as far as oil change intervals? Its different trany's right? Sorry I don't know anything about the sixxers.

Mine's and auto 4.0L 269 rwhp. The oil change intervals are the same as far as I know. The six is a sealed tranny but can still be serviced.

The manual transmissions are different from a 4.0 V6 to a V8. The 4.0 cars use a version of the T-5 (and not a very good one). It calls for ATF as a fill, but I'm not sure of the recommended change intervals. My V6 has 50,000kms on the clock, and the fluid has been changed a few times in a vain attempt to improve shift quality. So far, Amsoil's Torque Drive ATF (more robust than a typical ATF), and Pentosin MTF2 (lightweight gear oil like synchromesh) have been the best.