A friend of ours had a similar problem. The fan motor wouldn't work and he called a dealer. $100 to trouble shoot the problem; because the unit "was so old" he'd put in  a new one for $1400, it was his "best" advice.

We pulled the shroud and found a large wasp nest that was blocking the fan's rotation. We added some synthetic oil to the shaft bearings, cleaned things up and everything now works well. We found the top part of the cooling fins were bent, apparently from backing into a branch. With a small bladed common screw driver and patience, they were opened up and working again.

Total time spent was about an hour, with straightening the fins taking most of the effort. Cost $00.00. It operates like a new unit.

The shroud has about a dozen screws; an electric  drill with a #2 phillips bit takes them out in a minute or so. With the shroud removed you have full access to all the mechanical components of the AC unit. The motor and fan is in the aft part, and easily accessed, with clearance all around. If something is amiss, repairs are easily effected. It is an easy DIY project.

A dirt dauber nest on the fan would unbalance it and cause extreme vibrations.

There is a plastic tool called a "Fin Comb" that straightens bent a/c fins very quickly. 

less than $20 will save hundreds of $$ use it on your home a/c unit too.

http://www.bing.com/shopping/search?q=fin+comb&qpvt=fin+comb&FORM=HURE