Basel airport

Where are you flying from? Do you have checked in luggage?

Since it a TER to Colmar, it is what it is. Should make it, but if you don't, you just wait for the next one. Take the bus to St Louis station and get the train from there.

we do have checked luggage.

2 hours should be plenty I would think. I have been through Zurich airport with checked luggage and had to go through immigration and I was at the airport train station 35 minutes after landing. Unless you are coming from UK or Ireland, you will already have gone through immigration at your connecting airport.

We are going thru Heathrow as our connecting airport. Coming from New Jersey, do i go thru customs in London or Basel?

Just to be clear, Basel EuroAirport is actually in France and there is no train from the airport. So you will need to either take the public bus or get a taxi in to the city of Basel and on to the train station there.

London is not part of Schengen, so you go through both Immigration and Customs at Basel Europairport. Both Switzerland and France are in Schengen, so Immigration is the same, but customs are different, but I don't know the procedure, having never been there. I see there are separate exits for going to Switzerland and France. You want France.

The Distribus No 11 goes to St Louis station every 15 minutes and takes 10 minutes to get there. Since you are going to Colmar, that is the way you should go. Cost is 2.50 EUR for the bus.
https://www.distribus.com/sites/default/files/medias/files/2023-07/distribus-fiche-l11-110723.pdf

If you are asking this because you believe you need to buy the ticket for the train to Colmar in advance, then rest assured. This is bog standard regional mass transit. You just take a bus to st. Louis, and then a train. And the ticket you just buy on the day itself.
No need to worry about which train you will take. Trains go every half hour.