At the beginning of the season, I put a new 144" X 2" track on my ZRT by using rail extenders. Ever since, I have a vibration when the sled is coasting. The vibs go away under acceleration. Soon after the installing the long track, I lost the clutch side lower drive shaft bearing. I switched to 8 tooth drivers, so I don't think the track is hitting the heat exchanger.

What coul be causing these vibs?

Goes away when applying power, eh?. Can you get a feel for where it is coming from? I would look at the rear of the skid frame. When you are coasting the track is experiencing different forces and is under less tension, so I wonder if the the vibration is from track clips at the rear of the sliders, or something like that. If the track is hitting the front heat exchanger you should be able to tell by looking at the heat exchanger. The track won't balloon at the front under acceleration, only under braking.