I have no clue what size U-Joints im supposed to buy for my 1999 dodge ram 1500 4x4 for the rear driveshaft they have idffrent size bearing caps.

 

Get under the truck and use a set of calipers to measure the cross joint cap, or just take the cap with you to your parts store. I think there are three different sizes just buy all three and return the ones you don't use.

 

Rockauto.com will give you some listings. I was going to link but it won't provide a URL to the drivetrain/ujoint section.

napaonline.com

Go to a local autoparts store, better yet just call them.

otherwise...get dirty like merc said and put your own eyeballs and hands on it.