I had the same probglem with our 2005 Freestyle. My wife found a site on the net about freestyles www.myfordfreestyle.com had a question about air conditioning. There is a part in the compressor that cost me $40.00 and I had a shop put it in and the air is fine. Ford told me I had to replace the whole system for $1800. This is the site I went to to get the part. It is called a control valve. Cost about $180 for have the shop drain and recharge the system. Hope this helps.
If the compressor is getting weak, it will need to spin really fast, like at highway speeds in order to compress the refrigerant enough to cool the vehicle. Get a second opinion, maybe all you have is a clogged condenser, or you may just be low on r-134.