This split does not exist thus far. Mostly a tank with some sorceror buffs. Also, legal without an alignment change unlike a Bard/RDD/DD. The inability to get 16 AB for 4 attacks is really really annoying.

Sorcererous Defender

Sorceror 8 / RDD 10 / Dwarven Defender 22
Race: Dwarf
Alignment: Any Lawful
Playable: 1-40?, PvM (probably harder at lower levels until 9th or so)

Attributes
Str16 -> 34
Dex13
Con16 -> 22
Int11 -> 13
Wis 8
Chr12 -> 14

Level Guide

1 Sorc1 Knockdown
2 Sorc2
3 Sorc3 WF: Greatsword*
4 Sorc4 STR 17
5 Sorc5
6 RDD1 Dodge
7 RDD2 STR 19
8 RDD3 STR 20
9 RDD4 Blindfighting, STR 22
10 RDD5
11 RDD6
12 RDD7 Toughness, STR 23, CON 18
13 DDef1
14 DDef2
15 DDef3 Imp Crit: Greatsword*
16 DDef4 STR 24
17 DDef5
18 DDef6 Extend Spell
19 DDef7
20 DDef8 CON 19
21 RDD8 EWF: Greatsword*
22 RDD9
23 RDD10 STR 28
24 DDef9 Epic Prowess, CON 20
25 DDef10
26 Sorc6
27 DDef11 Armor Skin
28 DDef12 CON 21
29 DDef13
30 DDef14 Great Str I, EDR I, STR 29
31 Sorc7
32 DDef15 CON 22
33 DDef16 Great Str II, STR 30
34 DDef17
35 DDef18 EDR II
36 Sorc8 Great Str III, STR 32
37 DDef19
38 DDef20
39 DDef21 Great Str IV, STR 33
40 DDef22 EDR III, STR 34

*Choose one handed weapon if you want to go sword-and-board for and extra AC boost, but your HP and DR should make it more effective to use a 2-hander

Combat Vitals
Melee AB (w/ GS): 41/36/31/26
Damage: 2d6+18 (17-20/x2)
AC: 25 (mundane full plate)
+4 Defensive Stance (dodge)
HP: 660
DR: 21/-

Saves
Fort: 28
Refl: 16
Will: 24
+8 vs Spells

Skill Points: 99
Spellcraft 39 (40)
Lore 8
Discipline 43 (55)
9 spare

Spells
Known: 7 5 3 2 1
Per Day: 6 7 7 5 3
Level 1: True Strike, Mage Armor, Shield, 2 more
Level 2: Bull's Str, Endurance, Flame Weapon
Level 3: Keen Edge, Haste
Level 4: Improved Invis I'm not sure how useful Extend Spell will be with the spellbook you've laid out. The only two spells you might want to extend are Haste and Improved Invisibility. You can't do I.I. because it would become a level 5 slot spell, which you can't cast, and extending Haste would stick it into the only level 4 slot you have, keeping you from casting I.I. Maybe replace it with Lightning Reflexes; that reflex save is hurting.

Aside from that, my only recommendation is to take one more RDD level pre-epic and then take RDD 9 at level 21 for +1 skill point.

It's a nice build. Seems a good alternative to the Bard version for settings where alignment shifts are hard to come by.