I have a 2006 Specialized Comp XC running probably its original Shimano Deore RapidFire click shifters.

The rear shifter is jammed up, can't downshift anymore (won't "click"), probably gummed up and not responding to opening up spraying and lubing.

I should probably just replace, but having trouble making sense of what replacement will fit a bike this old.

current specs

• Front Derailleur Shimano Deore, bottom-pull/clamp-on 34.9mm
• Rear Derailleur Shimano Deore LX
• Crankset Shimano Deore, 22/32/44 teeth
• Bottom Bracket Shimano BB-ES30, 118mm spindle
• BB Shell Width 68mm English
• Rear Cogs 9-speed, 11 - 34 teeth
• Chain SRAM PC-951

Probably any Shimano compatible 9 speed would work. I think Alivio is the highest new stuff you will find.

If you have problems finding a Shimano replacement, Microshift makes compatible shifters. I've not used any of their shifters but did replace a front road der. for a 3 x 9 and have found the part well made and works fine. Link to one w/o gear indicator, but they also make one with an indicator, if you want. Marvo Xpress Shifters 2/3x9 | microSHIFT

Sometimes you can find older model shifters on eBay. The one you want is available new but from China. I've bought parts from China in the past when they were outrageously expensive here like a derailler hangar that sells for $30 at one store I frequent but sells for under $10 online. If they ship by "Speedpak" shipping it gets here reasonably quickly but if they use China Post it takes agonizingly long to arrive. Check out this Chinese seller who has the right side shifter which should be your rear shifter. Compare the picture with what you have now. Not cheap at $48 but it may match your left shifter https://www.ebay.com/itm/403049417498 The 9 speed version seems to be really hard to find in the US. A fair number used are on eBay from overseas sellers at reasonable prices like this pair https://www.ebay.com/itm/275856934079

Heavy caution that those shifters might not be real product. Ebay has no protection against fakes and knockoffs and stuff like that and the Chinese government and industry doesn't care about any of that.

This one will do the job, on the right click used and new and there's a unit for 21 bucks shipped.
https://www.amazon.com/SHIMANO-SL-M2.../dp/B086RXT6JZ

or go back to Deore
https://www.ebay.com/itm/256105022755?

Here's an "Altus" that will work for 15.99 and free shipping
https://www.ebay.com/itm/165964161580?

Remove the cable port screw (black with a slot for a Philips head) Spray liberally into the hole with WD40 and work the lever. Should start functioning normally after 1-3 applications and using the levers.

Don't use a heavy degreaser, it'll damage the plastics, WD40 is just perfect.

That would incorrect Shimano would use JIS screws, but in the end everything has moved to Cross Tip which is compatible with both (like JIS) but is the new international standard and simplifies things. However Phillips is always incorrect with Shimano derailleurs. Easy to cam out and ruin those little screws. The Park Tool DSD-2 is my favored screwdriver these days as it is Cross Tip and really nice feel but I have used Vessel in the past and have one kicking around somewhere.

Uh...ok. Black Phillips head screw aka something Vegan knows the name of.

Pic for reference.

Yes Phillips screws are cross tip but Shimano generally has used JIS which is also cross tip but slightly different. A JIS screwdriver is going to be more precise and will work on both but is designed not to cam out. These days though as stated above they have sort of gotten rid of both and went with Crosstip in the various sizes which combine the two.