'Well, those are some awful tight pants you have on. Where did you get 'em, like, Queers 'R' Us?'
The Office is typical for crude and slightly cruel jokes. In this one, the joke is based on a stereotype about gay men. The stereotype is that if you are gay (a man attracted to another man) then you wear very tight pants, and that if you are a man wearing tight pants, you surely must be gay.
When the character asks where the tight pants come from, "Queers 'R' Us" is an allusion to "Toys 'R' Us" which was extremely popular to kids toy store in the U.S. from the late 1990s through the early 2000s. Queer was historically a derogatory term for LGTB+ folks. The word has since been reclaimed by the LGBT+ community in some areas -- some folks like to use the word for themselves, so other's can't harm them with it, and take pride in the word as one to encompass all the community that is not an acronym. Other folks find the word has too much history and pain associated with it. Anyway, "Queers 'R' Us" is intended to be a sarcastic insult inside the joke. The joke, then, is one where the speaker of this joke is the one laughing, while the basis of his joke - gay men - bear the brunt of untrue stereotypes.
First guy says, "Well, I'm a astronaut, so I drive a Saturn." And the second guy says, "Well, I am a pimp, so I drive a cheap Escort." And the third guy says, "I got you both beat. I'm a proctologist so I drive a brown Probe."
This joke is based on puns and allusions. The joke escalates with sexual humor with the second guy, and then crude and sexual humor with the third guy.
The first guy either actually is an astronaut, or is the owner of "a Saturn," which is a car brand. If the first guy is the owner of a Saturn [the car] then if he makes an allusion to Saturn [the planet] he could claim to be an astronaut -- who might in the distant future travel through space to land on that planet.
With the second guy, he claims he's "a pimp" which means he's someone who manages a "business" of selling people (usually for sex) to other people. "escort" is a term that means a young (usually female) person who is hired as company for another person (either sexually or non-sexually, but often with the implication of the first, even if it doesn't happen). So, in this part, the pimp is claiming he "drives" "a cheap Escort". An escort is also a type of vehicle, so there's the pun. But the rest of the "humor" comes from the idea of the pimp "driving" (as a euphemism for "riding" or "having sex with") "the escort".
Lastly, a "proctologist" is a person who's job is in "proctology" which "is the branch of medicine dealing with the rectum and anus. There is an instrument doctors use called "a probe," which is put into the rectum and anus to get samples, and check that the body is healthy. There is also a vehicle called the Ford Probe. So this last joke the third guy "drives a brown probe". The pun is that it could be a brown Ford Prove vehicle. But what this third guy refers to is actually "driving" (as in "inserting") the medical instrument "a probe" up another person's butt. And the medical probe is brown, because of poop. Theoretically, there is also the insinuation of anal sex, which is also called "gay sex" because gay men do it.