I did the TC & tranny gear switch mods because my OEM TC lockout switch quit working... It was too much $$ at the dealership, so I bought a pack of diodes and a pair of switches for cheap.
In 3rd or 4th, I notice mine drops the RPM approx 400 (I say approx because the trail rig -'89 4.0, 4.56, 33" - has no tach) My 88 4.0 DD has a tach, but the tranny in it feels 'softer' and the OEM switched lockup is still noticable.
When I was DD the trail rig (all hilly 2-lane backroads with 3-8 miles between stop signs), I'd leave it unlocked from 1-3rd, then either lock it and ride in 3rd or 4th. Either way-TC locked or unlocked- if the shifter is in the OD position, I can downshift to 3rd with a flick...no need to give more gas to 'force' the downshift.
Off road is where these mods shine...lock in 1st gear and lock the TC for downhills = compression braking just like my 5-speed. Lock in 2nd gear for 'softer' starts or more wheelspeed on slick hillclimbs.
By your description of the tranny 'stalling', I'd say you are getting lockup on demand.
Find a steep hill, select 1st gear/low range and descend with the TC both locked & unlocked... you should notice the difference then
