I just made a very quick read of your description of the problem.
I agree that you very likely are experiencing desense because of the meg and a
half difference between one of your UHF TX and one Rx.
This is a pretty common problem when more than one system is located at a site.
A quick look at a generic Tx / Rx isolation graph shows at a meg and a half you
need about 85 dB of isolation at UHF. (assuming something like 25 watts)
Your antenna spacing (sounds like about 4 wavelengths) might be giving you 22
dB or so.
You are probably going to need to build a "window filter" which is similar to a
duplexer.
You might be able to use just a couple of cavities and notch the unwanted Tx
from your Rx if the cavities you use can notch that close.
You can also get some notching by using critical length cables to maximize
whatever notch & pass response the cavities already give.
You likely will need the help of someone in your area with a tracking generator
to help.
And with physically close spaced transmitters like you have, I would be a good
idea to get transmitter isolators to avoid problems in the future.
Keep in mind the numbers above are a quick rough estimate.
However you really are looking at something around 60 dB of additional
isolation no matter what calculation you use.
