I figure that plenty of members here own Ram trucks of this vintage so I thought I'd pass along a bit of info.
The 545RFE trans that backs the 5.7 HEMI is a decent trans but at 325,000 miles, I am about to dig into the transmission again to change the shift control solenoid.
The symptoms leading to this: Loss of 1st gear and overdrive as well as torque converter lockup. 2nd, 3rd and 4th work fine, as does reverse.
Fluid level was 1/2 qt low. Not enough to cause any trouble.
In July 2016 at 265,000 miles, I experienced the same symptoms. The MIL came up on the dash, the code came up a P 0750: Shift control solenoid. I did a search that offered a few YouTube videos on replacing the solenoid pack. The fluid gets drained, pan removed, then out comes both filters. The valve body comes down then the electrical connector is released for the solenoid pack. It comes apart easily and goes back together just as easily. In 2016, it was $240 with a $200 core from the nearby NAPA store. I never heard of a core on an electrical part like this but I went with it. Now almost 3 years later but only approx 60,000 miles, it has failed again. NAPA no longer offers the remanufactured unit. They have a new Echlin part for $319. The closest one is in Montana. I'm in CA. By next Friday I may have it for the weekend.
I checked with the local Dealer: $450 and it is backordered with no estimate of availability. With millions of these trucks on the road, how can that be?