Hi Ken - I still have your old '93 FZJ80 - it runs good but I don't take it out too far.... What year 80 did you get?
The MARS Mod on a Ham radio only allow you to listen (monitor) GMRS stations not transmit on GRMS. So most people that need both Ham and GRMS run two radios . Ham can be 2m or 70cm or both. Looks like you have a Ham radio that does both. As far as I know, the FCC frowns on a single radio that can transmit both Ham and GRMS. So you're probably looking at two radios if you want to transmit on both Ham and GRMS. For Ham comms use on a trail with other trucks on 146.460mHz, repeaters are not really needed.
To be legal, Ham needs a license from the FCC with a test, GRMS is $35 registration with the FCC, no testing. Most people use GRMS these days. Some people don't bother with a GRMS license but you'll likely need a FCC call sign to get access codes for permission required repeaters.
I run a GRMS 5w handi that can hit repeaters with an external antenna that works ok for trail runs (Wouxun 916, on sale for $60 at bettersaferadio.com) and its a superheterodyne (good selectivity) not a radio on a chip like the Baofangs (much cheaper). My GRMS is programed with a USB cable for the PC, most work that way although some are using Bluetooth these days.
For GRMS repeaters in AZ, go to myGRMS.com where they list repeaters by state. Some are free, others need permission to use.
FYI - around Southeastern AZ, GRMS repeaters in AZ/NM are listed on the the SWCRS.org (Southwest Community Radio System) you need to join to see the Repeater details but its free or very low cost. These guys link their repeaters from Tucson to Abq which is pretty cool. SWCRS does not cover North of Phoenix.
Again if you're just looking for point to point communications with the trail group, a repeater is not really needed for GRMS either.
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An entertaining video on how to use GRMS repeater is here:
and GRMS basics:
Good luck
GeoEng