Sergejs

Hi there! Fuel filter is located right in front of the engine on the left side. You can see it from the underneath of the car and cant miss it. Check, if it is not leaking on the sensor socket. It can be fuel pressure control valve as well.
Thanks, 

Torque converter is weak chain link on these transmissions. You will be taking it off together with the gearbox anyway. Besides, there could be inside some debris and metal particles too. If you don't want to take off transmission in near future again, I would strongly recommend to do torque converter as well. This is normal practice.  You cant open it yourself. You need to take it off and send to someone, who can professionally cut it, restore everything inside, weld it back and balance. I have contacts here in UK (cost me 250 GBP), but in Norway there must be some companies too. Yes, you need to wash somehow your cooler as well. You obviously cant use the water. I don't actually know how you can flush it properly. Maybe with diesel fuel, but it is messy job. Theoretically, filter should have been keeping all the metal particles, but you never know. There could be aluminium dust as well. Personally, I have replaced my original cooler with Chinese aftermarket one, which I bought on e-bay for 40GBP. I have attached it to the original cooler with clips, there is enough space. Have done about 10 th. miles and it is working very well, oil temperature was never higher than 80C. Had to modify pipes connections though. 

If metal particles are in the tray, this is not a good sign. Need to take gearbox off and look inside. You most probably have 41TE, A604 gearbox. Very popular gearbox in USA and Chrysler have stopped producing them only recently, 5 years ago or so. Plenty of second hand gearboxes available on e-bay, but I would recommend to put properly checked and reconditioned one. Another option is to buy second hand and repair kit with friction discs and to make one out of two. This is what I did. Also Torque converter need to be opened and reconditioned, if you want to do this job properly.
I have been through this process myself twice. Second time was my own fault as I haven't noticed faulty needle bearing. Gearbox itself is very simple, if you can follow simple procedures, there are plenty of information on You tube and in general with instructions and manuals. A lot of circlips to take out and put back.  
I'm in the process of reconditioning another gearbox unit for my Chrysler now, so, next time gearbox is broken, I have ready unit to put in.
Good question regarding using gearbox from petrol engine. I'm confident that gearbox from petrol 3.3 engine is the same as on 2.8 diesel, except gear ratios. It means that gearbox from petrol engine will work, but gears will change at different speeds (higher rpms). The only thing you need to check is electric connection on plug and pipe connections for oil cooler. All the fittings and everything else is the same. I have seen the post (possibly on this website) that guy did it on his diesel voyager and it worked well and he was very happy.
Good luck with your project!