Hey raggie,
There are so many different things that could be hosed up. You could certainly be low on refrigerant, but first thing I'd make sure that your coil is clean and that your filters are clean so that enough air is getting through it. Since you measured the temp in there I assume you can see the coil OK and make sure it's not clogged with dust and other nasties?
in most places you cannot buy refrigerant without a license I think, and you probably dont have access to the proper tools to weigh it in properly, not to mention pumping down the old system and recovering the r22 or whatever it is in there.
First thing I'd do is to wash the outdoor coil with a hose and get as much of the muck off that as possible. Unfortunately if your unit is a 2 layer coil then it really has to be taken apart and the coils separated to get the garbage out of the middle, and if you have something like a cotton wood tree nearby then it will be totally clogged. Then I'd clean the indoor coil as best as I could. If it's really badly clogged you can pick up some coil cleaner at a local supply house. That stuff is really nasty though, it's an acid or something that actually dissolves the hair and crap on the coils, you have to rinse it off immediately after it's dissolved the goo or it will eat the coils too. Oh, and don't use a pressure washer to wash the fins /ubbthreads/images/graemlins/smile.gif They are delicate and will all bend completely flat closing off any airflow that is left! I've seen pictures of a friends place who hired a pressure washing company to clean his house and cleaned his AC also, completely destroyed the coil.
if none of that helps then you're going to have to call somebody. It would still be cheaper to have someone come out and recharge the system than to replace the outdoor unit with a new pre-charged one. I know that you used to be able to get pre-charged line sets that you just screwed on, but I don't know if such a thing is still available. And even if you could, you'd have to have someone out to recover the old charge anyway.
but it could be a number of other things too, like a kinked line set, or a clogged filter dryer, or your outdoor fan might be running slow or not at all.
Good Luck!