Changing the rear end is a band aid.
As already mentioned, the cam that is installed is your biggest handicap.
That 3.21 rear end keeps the rpm down at freeway speed, that's a good thing.
Without an overdrive, you will regret changing the rear end because your sluggish launch will still be there.
When the rear end doesn't satisfy your goal, a complete rebuild will happen and now you won't like your gear ratio
I have 3.55 sure grip for my 3100 lb dart, the correct cam for getting out of the hole,lower planetary gear works out to 3.98 in first gear. It will get out of the hole but I'll pay for it on the freeway. An overdrive unit is being planned for the future when funds permit.
Change the cam.