96 2WD, Trouble shifting into 2nd

[Salmaxum]

Have a 96 XL 2wd, 4.0, 4R55E with 133,000 miles, that won't shift into 2nd - though, I can force it to shift into 2nd by taking my foot off the gas and letting the rpm's drop to below 2,000 rpm and then giving it a little gas it will always shift into 2nd. Once in 2nd, it has no problem shifting into 3rd or 4th. There is no slippage, shifting is smooth above 2nd, acceleration is fast & strong, leading me to feel that there is plenty of life left for this engine & transmission.

I've taken it to 2 shops, both advised for a rebuilt tranny, one suggested that it might be just a sticking solinoid. My first order will be to replace the transmission filter and flush ATF (using Mercon V) to see if that helps any. A neighbor will help me read the DTC's (Check Engine light is on & O/D light is flashing) as soon as he can borrow a scanner.

Since I belive the transmission is still in good shape, I'd like to fix this problem myself, hopefully, without having to do a complete rebuild.

Does anyone know what might be causing this problem and what steps I could take to diagnose.

Could someone explain why taking my foot off the gas and letting the rpm's drop to below 2,000 rpm and then giving it a little gas allows it to slip into 2nd gear?

Also some advice on tech resources, I was planning on getting a subscription from Ford to access their service manuals, etc. - do you suggest buying 4R55E repair manual from ATC? Would it silly to subscribe to Alldata too if I were subscribing to Ford?

 

[Glacier991]

I'm going to guess that the solenoid that controls 2nd (SSB) is somehow acting up or partially plugged. There could be other valve body issues that come into play here too... as fgor the foot on/off... maybe a surge of hydraulic pressure causes the shift thru a p[artially plugged solenoid? I'm somewhat guessing here, I'll admit... but the SSB solenoid is the only one activated "on" during the shift to 2nd.