Repair shop said i need a new alternator in my 2002 olds Bravada ($371 installed) I have changed brakes on a vehicle before - could I replace alternator myself? Is it difficult?

Replacing brakes is nowhere near what's involved in an alternator replacement. I suggest if you decide to give it a try, first get yourself either a chilton or haynes manual to guide you through the process.
Neither manual is very expensive and you get to keep it to help with other future repairs.Most larger parts stores sell at least one or both of those. Difficulty level depends upon your skill and how good the tools you use are. One tool tip....use six point sockets and wrenches unless it's absolutely impossible to position them correctly. Twelve point ones tend to round off bolt heads when they are very tight.
Good Luck!!

It's not that difficult. Loosen the tensioner remove the belt off the altenator remove the bolts and wires from the altenator. you can do it.

I have replaced several alternators in my life. Generally the replacement process is easy; however, this depends upon the accessability of the alternator itself. The attached link clearly explains the needed work. A new altrernator will run ~ $100.

http://www.youtube.com/watch?v=FiJoV5SsrYs

The FixYa Team.