You need to keep in mind that most of the people on the show are local detectives assigned to the task force. They are sworn as Special Deputy US Marshals. The USMS provides them with a portable VHF radio which is usually an XTS. If you are seeing other radios on the show its becuase they are using their local agencies radio when inside their system coverage area.
Also, this show does not show a lot of the investigation, it just shows them banging doors. It most certainly does not nor will it ever show them tracking cell phones. Google the word "triggerfish" and you will see the best tool the USMS uses. Ive seen this system in use several times. You can turn your cell phone off and they can still track your location almost live using triggerfish......with a court order of course.
The majority of their tactical radio traffic is simplex P25 and is usually encrypted. Almost all other comms are over nextel and phone.
If you would have watched the new season at all, you would know this is not the case.
Through multiple episodes in different locales and states, there were seen using Kenwood portables on UHF. Of course cell phones were used, but quite noticeably so were these portables.