I have a really hard time singing back up while playing bass. When I play rhythm guitar I can sing a long pretty easily. But I'm a bass player. Any suggestions besides practice?
Nope. You just need to practice. It's difficult at first.

Well, I lied; I do have a tip: Make sure you can do both individual parts perfectly without even having to think about it. In order for you to do both together, you can't focus too hard on either of them.
Only play what you hear. If you don’t hear anything, don’t play anything.
-Chick Corea
Therein lies your issue bro. Its quite easy to strum on a guitar if your doing an easy song (I'm assuming this). The bass on the other hand requires that you only hit one string at a time in most places.

Try simplifying the bass parts to the bare essentials and then sing with that. Slowly the parts back in as you become more comfortable. Shouldn't be too bad. I sing/scream and play bass in a metal band. If my uncoordinated ass can do it so can you!