GM-GMC: 1991 gmc 2500 series, limited slip differentials, friction modifiers

Question
What does the differential fluid do? Where is it in the truck and can i check the levels myself?

My truck seems to chug and hard to pick up speed .... would this be due to a possible clogged fuel filter or a bad fuel pump? I seem to get slight gas fumes in the cab....the gas tank was recently changed as well as a tune up.

I just had the U joints replaced front and back.  Would an improper instalation cause this chugging as well?

Your answers would be appreciated.

Answer
Differential fluid lubricates and cools the differential, some have friction modifiers to prevent clutch chatter in limited slip differentials.  As for U joints.  Improper installation could result in a vibration or driveline shudder, usually during acceleration but would not likely cause poor performance/acceleration or engine "chugging"  and for the fuel smell you have you should return to the shop that installed your new fuel tanks and ask them to have a look at it.
Hope that helps.