Hi James,
I faced a similar dilema 3 years ago - I had a 98 SWB shogun and had it converted to LPG.
It performed well and I was pleased with the saving at the pump, it reduced running cash flow considerably.
I passed it accross the family to my son as I found that a SWB was not the best towcar niether did it have enough room for my dog - had it been a LWB I would still be using it.
I now tow with a LWB desel Isuzu which is excellent but admit to a hankering back to a nice smooth V6 or V8 petrol/LPG.
I think in your case you need to consider pay back time firstly and then wether your Ford is durable enough to lastfor the next three years without major expense - at 50k it should be in good fettle but only you know what condition it's in.
I have no experience of US Fords but a soreexperience with Jeep and Chrysler - if an Explorer is in the same league as a Cherokee reliability wise then forget it - you might as well pour yourcash down the loo!
Against that you have to consider an additional investment of around