the declination lock won't lock any more...
is my only choice to send it back to the factory (booo...)?
Unfortunately, you will probably have to send the unit in for sevicing, to restore this function. This type of scope is a highly precision device, and parts can wear, causing a loss of certain functions. I wouldn't recommend trying to take the unit apart, unless you're a super mechanical person. Even if you did, you'd have to identify the worn or failing part, and that would be a challenge without the tools and replacement parts they have at their repair facilities. Some cities have "sky clubs" and some of the people are avid skywatchers, and might know something about this particular problem, and may be of help, but that might your best alterntive to having it serviced right away. Often times, a switch or part can get slightly compromised by dust, or lint, and require a minor cleaning.
Here's Meade's Support Site, in case you don't have it:
Good luck, and hope this helps. Read More At : ETX-90EC (325 x 90mm) Telescope...