Encoders for a Mecanum Drive

I realize people have talked about equalizing motor speeds before, but this seems a bit different to me. With the way our chassis is set up, we have a lot of weight in the back, and very little in the front when not carrying any totes. This results in the front two mecanum wheels lifting off of the ground when driving which, as you can assume, results in some pretty weird forward/reverse, strafe and rotation commands. I’m wondering how we can use anything (maybe PID?) to set every wheel equal in speed regardless of contact with the floor and weight on the wheel. Thanks.