Can somebody confirm whether the rollover vent valve is replaceable on a 99 GMC Jimmy or do I have to replace the whole tank?

 

What is a rollover vent valve?

If you are talking about the valve at front of tank, and plastic tank with valve 'welded' in.. read this:
https://blazerforum.com/forum/2nd-ge...s-fixed-63397/

Otherwise probably tank

 

if i remember correctly the vent tube is in the fuel filler neck seprate from the tank. so yes it is replaceble.