2003 Suburban 4x4

It has been sitting for several months. Got a new battery in it and started right up and idled fine. But when I put it in drive wont move just revs up. Got it to move only in 4low.
Any ideas? Ran fine when parked. Maybe a tranny filter clogged from just setting?

 

Did you check to see if the fluid was up to the correct level? A clogged filter is possible but I would check the level first.

 

I would check the fluid on the transfer case also