There are several options.
Mears is 1. For $15 RT PP, they will pick you up at your resort, and return you to your resort. There may be intermediate stops along the way.
Dial "0" from your resort phone (or any Disney house phone) the night before your wish to leave.
Have the Disney operator transfer you to Mears.
Tell them you wish to go to Universal, where you are staying and approximately what time you wish to leave.
They will give you a pickup time and location at your resort. Be there about 15 minutes early.
Pay the driver $15 PP cash.
When you arrive at Universal, you can arrange a pickup time for the return at that time, or call the number they give you on the return voucher two hours before you wish to leave.
Then be back at the same location you were dropped off at for your return.
A Taxi is also an option at about $30 each way.
A 1-day rental car from the Dolphin or Car Care Center from National/
Alamo is also an option from as litlle as $25 and up.
I just realized something, you mentioned a concert. If it is a daytime concert during park hours, Mears will work. They have several pickup times in the morning, and several pickup times near park closing. If it is an night time concert at Hard Rock, then Mears may not work. You will have to be at Universal most of the day as the last pickup time is not much later than 11:00am. Their last return to Disney is only about an hour after the parks close.