Both 4.7L trucks use the 545RFE (45RFE) transmission. As stated, look for TCM fault codes with a capable scan tool, not all codes will light the dash 'ck eng' light and not all OBD II engine code readers will read transmission codes. The latest and greatest PCM/TCM software revision followed by a quicklearn is always recommended.
P0740 may set with a hydraulic, mechanical or electrical torque converter clutch problem, but a scan tool may also tell you the reason 'why' it is disabling lock-up. False temperature and speed sensor signal information are 2 reasons for hunting or intermittent operation in an otherwise functional TCC (torque converter clutch) system. Since the speed and temperature inputs are deemed as real by the control module, it may not set a fault code for an 'unlocking' TCC.
The control module looks for an appropriate RPM speed drop when the TCC is activated. If it doesn't see the speed drop or it doesn't drop enough, the fault code is set.
It doesn't always have to boil down to a transmission rebuild, but front pumps and torque converters may be replaced as a pair if wear has occurred. Wear here could cause a hydraulic TCC turn-on leakage.