So my friends are convinced that my rear end fluid is leaking, quoting "grease" on the tailgate and bumper, but when it's dry weather, it comes off like dirt. When I'm parked, there are no traces on the ground of a leak. How would I know for sure if there is a leak (like, how would it drive differently)? And what would I need to fix it?

 

unless the axel itself looks 'wet' it aint leaking.

look at your blow by tube and see what it looks like, if you have alot of sludge comeing out check your oil level. you may have a leaky injector.

 

greaseable ujoints? will do that if over lubed. More likely its road grime.

 

Do your friends ahve an abnormal amount of water in their brain?

 

Possible, they're both Chevy guys (95 truck and 99 camaro), ha ha