I concur with Chris, check the ATF fluid as per the owners manual.
(engine warm, running, neutral etc)

If your installer added a tranny cooler, you need more ATF than the
stock setup.  Low ATF will definitely make it slip.

Does it slip all the time when cold?
i.e. how about reverse going up hill?  forward going up hill?
(going uphill puts a nice load on the tranny, slipping will be more obvious)

>never ASSume anything!!!   ive gotten vehicle back from the shop before with
>filler caps left off, wires left disconnected, you name it, major mechanic
>f*ckups....
>
>check the ATF and diff fluids NOW!!!!