Trans slip or rear end?

[98 Z/28]

every time I try to mash the gas it feels like the trans is slipping or my rear is slipping. The question is what should I start to look at and should I be worried about expensive repair costs?

[sunsetorangess]

Hows the tranny fluid look? Level?
J

[98 Z/28]

It looks OK

[MnLS1]

sure its not the clutch

[98 Z/28]

It's an a4