You might be able to find a local place to do the cleaning.
Also if you have a compressed air source its not that hard to hack up your own home brew injector cleaner. Basically you just need to put some voltage on the injector to switch it open and closed.
An old D cell flashlight housing works for this, run wires from the bulb connections to the injector pins and use the flashlight switch to open and close the injector.
Get a long piece of tubing that fits on top of the injectors, fill it with carb cleaner, connect compressed air to the top of it and use the flashlight switch to open and close the injector.
There are youtube examples of doing this, search "home made fuel injector cleaner" and you'll hit them.
Don't use a flammable solvent for this though, it might go boom.
I didn't answer your post just to try to sell you some injectors but I do have a set of pink 21# injectors that were cleaned and flowed by www.mrinjector.us
They ran great on my 02 GT when it just had bolt ons and I also had a Bama tune. I put only around 1000 miles on them and then switched to a KB with bigger injectors.
The pink 21's came on 02-04 GT's but can be swapped in to the 99-01's without needing a re-tune.
I'll let them go for $110. including shipping, they come with the test flow sheet from mrinjector.
Send me a PM if you are interested in them.
All that said, the detonation you are getting could be dirty injectors but like Dawson said it also could be carbon build up or something else.
Take a look at your plugs and see if they have black deposits on them.