For Overdrive tranny check out these guys. Lotsa good info there even if you don't buy one of their units:

www.bowtieoverdrives.com

Overdrives are really a best of both worlds deal, can't beat 'em.

I agree with Richard on the Explorer 8.8, they are very available, most are positration, have good gear selections with 3.55 and 3.73 and they come with disk brakes. If they are the right width for your ride, it's a sweet deal. They are also plenty strong.

Pat