disco gearbox issues, auto

the Disco (Discovery 2 V8) has been sat for three weeks while I changed the heater matrix. I've put it all back together and it starts first time. my question is this.

It revs ok, but I tried to drive it and it wont go about 1500 revs and it crawls only. at first i thought I was in low range but sadly this is not the case.

am I right in thinking its the gearbox oil?

am not familiar with auto boxes mate but i dont think there would be any harm for suggesting a blocked filter in the box?
does the engine rev freely in "N"? just to eliminate gear box and engine trouble?

yeah engine revs ok in Park or neutral.

I think the fact that the car has stood for 3 weeks means that the oil has drained, on starting the car I am thinking that maybe the oil is too thick to get through the filter thus not working correctly?

as i said it was a shot in the dark, but i also think it could be engine wise and it just struggles under load?? just take out the crossmember under the box, remove the inspection panel and filter, replace gasket while your there for only a few quid and see how it goes, usually on my jags when the filter requires changine and fresh oil they have not been changin gear soono enough thats how i knew to change it but my lannys will always be manual so aint touched an auto box on one of these. give it a go and get back to me if yer find anything, am suprised no one else has helped yer out here to be honest

Cheers mate.