Today my 04.5 400 sportsman's transmission started slipping, it makes a horrible ratcheting clicking noise like a chain skipping over sprockets. It only happens under hard load and occurs much more in low range and reverse. When there is not a heavy load it will not occur. I looked at the parts diagram and there are 2 chains in there, is there a way to tighten the tension? Is this a common problem, I have about 3000 miles on it.

 

Wasn't a common problem at the shop on the larger gear cases,had more problems on chain slack,jumping teeth on the single "silent chain" gear cases,BUT from your description this sounds like what's happening. A lot of it depends on how hard it was used,pulling,plowing,etc.
There is no adjustment. If it get worse the gear case will have to be split to replace them. If you have only one really bad chain I would still replace both chains. No use having to go back in the gear case later on. OPT

 

Thanks for you help, do you know what all has to be removed to get the transmission out, where is it removed from back, either side etc...

 

Basically the whole rear end has to be removed, drive shafts swing arms,shocks,along with pulling the clutches,clutch backing plate,etc. Look here on your model under gear case and rear suspension. The gear case only has 4 bolts underneath and a few on each side of the frame holding it,but you have a lot of parts around it that have to be removed. Browse OPT