Is there a feat...

[ForceUser]

That seems a bit much to me. Part of those creatures' CR is based on their immunity to sneak attacks and criticals.

It hasn't led to a screaming horde of players clamoring to play rogues, so I figure it's not broken. I use a lot of big, crunchy monsters as bad guys---vampires and frost giants, for instance, are the campaign-driving villains IMC right now. What's a rogue going to do against a vampire in a standard game? Hurl harsh language? Buffy got me thinking, is all.

I'm not too worried about CR balance issues. If the group punks my monsters, great! That's what heroes do.

 

[Johnny Angel]

So, it's:

A ring would cost Spell Level x Caster Level x 2000 (for constant effect) x 4 (because the base spell's duration is measured in rounds).

8000 gp - Ring of Grave Strike
8000 gp - Ring of Golem Strike

At the levels where undead and constructs start becoming more and more of a nusiance that's not a terrible price to pay.

 

[Nac_Mac_Feegle]

Well, if you're just looking for find traps, you can do Rogue for 1st level and the Ranger from then on out. Your Disable Device will suffer, but Search is a class skill and as others have suggested you can take undead as your favored enemy. You can also ask if your DM will allow the Skill Knowledge Feat from Unearthed Arcana and take Disable Device and Open Lock (or Tumble) as Class Skills for your other class levels..

If you multiclass, I thought class skills for any class you have a level in, become permanent class skills anyway.

Just checked the SRD

Skills: If a skill is a class skill for any of a multiclass character’s classes, then character level determines a skill’s maximum rank. (The maximum rank for a class skill is 3 + character level.)
If a skill is not a class skill for any of a multiclass character’s classes, the maximum rank for that skill is one-half the maximum for a class skill.

Feegle Out

 

[Staffan]

The maximum ranks for any skill which any of your classes have as a class skill is character level +3. However, you must still pay the cost according to the class you're levelling up in. So a rogue 1/cleric 5 could have 9 ranks in Search, Open Locks, and Disable Device if he wants to, but he has to pay double if he wants to raise them with his cleric levels.

 

[Celtic_Bear]

Bow Down Before My Obscure Knowledge!!!!

There is a feat that does EXACTLY what you are looking for:

Finding the Secrets
(Kalamar Players Guide)

Prereqs: Knowledge (Monsters) 6 Ranks [non Kalamarans could substitute Knowledge (Religion) for Undead and Knowledge (Arcana) for Constructs and Knowledge (Nature) for Plants], BAB +9, and Improved Critical.

Effect: It allows you to critical one category of monster, can be taken multiple times for different catagories.