The variable idle does point towards vacuum. Need to check the modulator for leakage. if it is leaking, it can cause the extremely soft shifts.

1st Modulator
2nd cracked and leaking vacuum connectors (from the sitting)
3rd k1 and k2 spring kits (firm up the shifts) You could also get the superior shift kit with tons of springs
4th Internal tranny issues

This is the order in which I would work on slipping 2-3 3-4

At that mileage I doubt your clutches have worn out, slipping due to other issues will speed up the wear and fill your fluid with clutch material which is hard on seals, clog the filter ect.

__________________

77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg