so we spent saturday continuing the troubleshooting here based on some input here as well as some discussions with Trevor.
Testing voltage at batt (replaced again saturday as the bosch had died again), tested at Alt (engine on and off). both were good
Tested voltage at starter relay in kick panel at hot side as well as ignition side during crank. All seemed correct there (i found it odd that the voltage rapidly drops as soon as the test probes are grounded). Also swapped in the relay from a working D2 with same results)
Voltage at coil is good and spark is good.
ran truck for 20 minutes around the farm, came back and shut it off. wont start.
tested voltage immediately at starter relay, seems to be the same. tested voltage at starter, seems good. starter is knocking on ignition, just not cranking. We were able to duplicate this 2 more times (after it sits for 15 minutes, it'll start right back up again).
Of note, the starter solenoid becomes extremely hot after running, moreso on this truck than on my '95. possible issue? the heatshield is there though.
Have a rebuilt starter we're going to swap in this week. Just wanted to keep the log here incase anyone has any other input.
Thanks to you all and Trevor...hopefully it'll be rally ready in time.