4.0 Liter Thermostats

My '00 Grand Cherokee is running, IMO, way too hot, between 210 - 240 after 10 min of driving. Temp gauge pegged the other day in traffic. Local mechanic says I can't put in a colder 'stat because it'll cause the computer to malfunction.
Also, as the temperature climbs, the oil pressure goes from marginal to really bad.... Like 8# at hot idle, and about 20# at 1800 RPM.

My attitude is that the computer works fine while the engine warms up, what's the difference if it only gets to 180*?

SOOOOO.............
Can I put in a 160 or 180 and not cause problems?  

It is critical that the engine run at the proper operating temp. If the PCM detects that it is taking too long to warm up, it will turn on the check engine light. All of the fuel system and emission system control data that is stored in the PCM is dependant on a 210 degree operating temp. Your car will not work correctly with the wrong thermostat.

A cooler thermostat will not make the engine run cooler if it already has an overheating problem that is not related to the thermostat. So, if you put a new 210 degree thermostat in and it still runs up to 240 or hotter, you have a problem somewhere in the cooling system that a cooler thermostat will not fix. It should run close to 210 all the time except maybe pulling a trailer up a steep hill on a hot day. But, even then 240 is too hot.  

I totally agree.  Start with a stock replacement thermostat.  If you are still running hot, then a flush may be an order. Did you checked your fan.  I don't know if its a clutch fan like the wranglers or an electric.  
99tj/ 4.6 stroker / 4.88 / '06 rubicon axles / 7"rockkrawler l/a / steering upgrade / agr pump psc box and hydro assist / mm 10.5k hyd. winch / 35" Trxus MT / comp cams xtreme 4X4 cam

Thanks for the suggestions. Being an old-school kind of guy, all this computer-controlled stuff really bugs me. I always used to carry a toolbox with a bunch of spare parts just in case, and did a lot of my own work, but with the new vehicles I don't feel like I have any hope of diagnosing or fixing anything.

I retired my last Jeep at 230K. It was a poorly-repaired salvage when I bought it at 140, and I was able to keep it going with almost no visits to repair shops, but everything was just about worn out. I just bought this newer one at only 80K, and I'm already beginning to hate it.