It would be the 545RFE.
For '05, the Ram had the following automatic transmissions:
45RFE, 545RFE, 48RE.
How to decode Chrysler trannys: we'll take 48RE. 4 is the # of forward speeds. 8 is the duty rating (strength). R is for rear-wheel drive (L would be for longitudinal, FWD). E is for electronically controlled (FE stands for fully-electronic, old transmissions used H for hydraulic). FYI, the first "5" in 545RFE is the # of forward gears.
(It's beyond me how the 545rfe can have 5 forward gears, and only 4 forward speeds. Yes, it has a second-2nd gear [to aide in passing grandma on the highway], but so does the regular 45rfe.) Oh well...