The enclosed driveline, or torque tube, is really a quite effective design — It’s advantages are positive location of the rear end, with no Panhard bars, traction bars or four-link setup needed for rear-end stability.
Only one U-joint is needed, and it runs in a constant lube shared with the transmission.
The primary complaint about an enclosed torque tube driveline is that there are very limited ratios available for the rear end. Additionally, the U-joints are expensive and more complicated to change than open driveshaft U-joints, and the seal which retains the lube in the tube, when worn, can cause all of the transmission lube to drain back into the rear housing, and eventually into the brake drums.
When everything is up to snuff, the torque tube is absolutely adequate for its intended purpose. When it wears out, or when the intended purpose changes, it may be time to convert to an open driveshaft.
To convert torque tube suspension to a open drive-shaft and rear-end, you’ll need to source a rear-end between 58 and 60 inches.
Common sources for applicable rear-ends: