Dodge Diff

I have been doing up an AT4229 for a some time now, I drive it every day. It has a 318 out of a 6t cab over Dodge and a 904 auto, very cruisy. Problem is to do 100k's i'm pushing 3000rpm a bit high. I was thinking about changing the diff? I have no experience in this. My AT4 manual states a standard ratio of 4.1 to 1, I will count the turns this weekend. My questions are, are these a 8 3/4 diff?, can I swap the internals with any diff that size like a dodge car diff 8 3/4 with a ratio around 3 to 1. And is there a way to tell if has had a Dana put into it with out removing the cover. Thanks

hi dodge AT 4 .I believe the diff's in the AT4 and the D5N are a Dana 60 The 1960 model's and before have 8 3/4 '' diff.The difference's to look for is the dana 60 that is in the 200's looks like a octagon shape housing and the 8 3/4" is like the old banjo type diff in a holden but bigger and a rear removable cover . If it's a 8 3/4 '' runaway ! I 've just spent 5 month looking all over australia for one . I found two. One good and one rebuildable. With the AT4 a ford f250 diff should bolt in to place or check out ebay for dana 60 crown wheel and pinion . I have a 1960 233 and a D5N 300 both with 318 on gas.
the 233 has a 4.88 to 1 great for hills on 7.50/16 tires. The D5N run to the best working out a 3.75 to 1 on 6.50 / 16 tires great out on the flat ground do'nt change in to top till 80 kms / hour but towing it's not great in to a head wind . So if you just cruzing on the open highway it fine but around town in the 50 kph zone needs to be in 3 rd at 2000 rpm . so have fun with the great beast !

You could also try a 518 (4 speed 727) out of a Dodge ram, i have one in my valiant and it drops 900rpm in overdrive.