"Revs while parked well"
How about when you stomp the %#*% out of it and hold it WIDE open. If it revs good--like the motor will blow up if you stay at it-- then look for something else- rear end locking up?, trans?, brakes?, wheel bearing?, etc.
rear brake was locked up some but we’ve fixed that. Axle spins fine. Unless it’s a 1:1 rear gear it’s not the gearing robbing that much power on stock wheels and tires.