Okay, now remember, RAMBLER is the manufacturer's (AMC) car line name, not a model! There were different model Ramblers. Sounds like you have an American, but they changed over the years. So what year and model? That helps! A Ford Ranger or GM S-10 pickup rear axle fits the American real good. Tread was 55" for 58-69 models. Rather than a narrower axle, try deeper offset wheels. Sounds like you want wide ones on the back, narrower on the front.
All the salvage yards list the parts separately because they can sell them that way. Call and ask for the price of a complete rear end, brakes and all. They will usually give you a break on that because they don't have to handle the individual pieces. If you don't mind pulling parts yourself see if there is a Pull-A-Part or something similar near you (www.pullapart.com
-- southeast and some midwest). They list prices on-line. Last time I got a rear axle (a while back!) they charged the listed price for an axle assembly (around $75 -- everything but the springs and shocks!).